SHALER^ 


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GIFT  OF 
1:  SEELEY  W.  MUDD 

and 

GEORGE  I.  COCHRAN     MEYER  ELSASSER 

DR.  JOHNR.  HAYNES    WILLIAM  L.  HONNOLD 

JAMES  R.  MARTIN         MRS.  JOSEPH  F.  SARTORl 

to  the 

UNIVERSITY  OF  CALIFORNIA 

SOUTHERN  BRANCH 


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UNIVERSITY  of  CALIFORNIA 

AT 

LOS  ANGELisS 

LIBRARY 


Digitized  by  the  Internet  Archive 

in  2008  with  funding  from 

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ASPECTS  OF  THE  EARTH 


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CaRon  of  the  Colorado, 
(^The  wider  benches  show  the  successive  stages  of  downcuitinK  of  the  stream.) 


Aspects  of  the  Earth 


A  POPULAR   ACCOUNT  OF  ,SOME  FAMILIAR 
GEOLOGICAL   PHENOMENA 


N.    S.    SHALER 

PROFESSOR   OF   GEOLOGY    IN    HARVARD    UNIVERSITY 


ILLUSTRATED 


NEW   YORK 

CHARLES    SCRIBNER'S   SONS 

1889 

r^  o  r 


93512 


Copyright,  1889,  by 
CHARLES    SCRIBNER'S    SONS. 


Press  <if  J.  J.  Little  u.  CV 
Astor  Place,  New  York. 


3  5  %'^ 


PREFACE. 

The  greater  part  of  the  several  essays  contained  in  this 
volume  have  already  been  printed  in  Scribncrs  Magazine  ;  the 
last  chapter,  that  concerning  soils,  has  not  been  before  printed, 
and  to  the  others  considerable  additions  have  been  made  since 
they  were  published. 

Something  in  the  way  of  an  apology  is  due  from  any  writer 
who,  in  this  day  of  excessive  book-making,  reprints  papers 
which  have  appeared  as  occasional  publications.  It  may  be 
said  in  favor  of  the  republication  of  these  essays  that,  although 
not  originally  designed  for  a  book,  they  were  written  with  a 
distinct  purpose  and  have  a  certain  common  quality.  They 
were  intended  to  give  the  general  reader,  unacquainted  with 
the  details  of  natural  science,  a  comprehensible  account  of  some 
of  the  more  interesting  series  of  the  actions  which  affect  the 
surface  of  the  earth.  For  this  purpose  subjects  have  been 
selected  which,  from  their  nature,  commend  themselves  to  the 
attention  of  intelligent  people.  In  treating  these  subjects  an 
effort  has  been  made  to  show  the  relation  of  natural  forces  to 
the  fortunes  of  man,  and  thereby  to  secure,  on  the  part  of  the 
reader,  the  interest  which  belongs  to  matters  which  affect 
human  welfare  alone. 

Those  long  engaged  in  the  study  of  nature  acquire  a  deep 
and  abiding  interest  in  phenomena,  however  separated  these 
may  be   from   the    ordinary  experience   of  men,  or  apart   from 


viii  PREFACE. 

common  subjects  of  thought.  Now  and  then  a  youth  may  be 
found  who,  by  some  odd  chance  of  birth,  has  almost  from  the 
cradle  a  passion  for  scientific  pursuits ;  but  with  most  the  taste 
for  nature,  beyond  the  mere  admiration  of  the  beautiful  things 
of  the  world,  is  slowly  acquired:  they  must  proceed  gradually 
by  way  of  the  matters  which  are  of  familiar  experience,  and  in 
a  measure  connected  with  their  sympathies,  if  they  are  to 
attain  the  naturalist's  spirit.  It  is  for  these  very  human  and 
fortunately  very  numerous  persons  that  these  essays  were 
written. 

Clearly  to  present  the  facts  of  nature  to  the  ordinary  reader 
demands  the  use  of  abundant  pictorial  illustrations.  No  con- 
trivance in  the  way  of  words  alone  will  effect  this  purpose. 
In  most  popular  works  on  science  there  is  an  effort  to  meet 
this  need  by  diagrams,  which  at  best  convey  to  the  mind  unfa- 
miliar with  the  matter  an  imperfect  idea  of  the  facts.  It  will 
be  observed  that  all  the  illustrations  in  this  volume,  except 
in  cases  where  diagrammatic  presentation  was  imperatively 
required,  are  taken  from  photographs,  which  have  been  admi- 
rably rendered  by  skilful  engravers.  In  this  manner  the  stu- 
dent has  a  more  foithful  presentation  of  the  actual  appearance 
of  the  phenomena  before  him  than  is  given  in  any  other 
work  of  a  popular  character  which  is  known  to  me. 

Every  teacher  who  has  had  occasion  to  inquire  into  the 
state  of  mind  of  students,  who  have  begun  their  study  of 
nature  from  books  in  which  the  ordinary  characters  of  natural 
phenomena  are  used  for  illustrations,  has  lamented  the  errors 
of  understanding  which  such  pictures  cause.  The  well-trained 
engineer  can  use  even  rough  and  sketchy  diagrams  with 
profit,  for  he  has  a  context  in  nund  which  serves  to  complete 
the    conception;     but    the   beginner    should  have  the  natural 


PREFACE.  IX 

object  which  he  is  called  on  to  consider  before  him  ;  or  if,  as 
is  often  the  case,  this  cannot  be  done,  he  should  be  provided 
with  the  best  possible  picture  of  it ;  a  representation  which 
will  be  reasonably  complete  without  calling  on  him  for  any 
previously  acquired  knowledge,  which  he  cannot  fairly  be  sup- 
posed to  possess.  From  the  point  of  view  of  the  instruction 
wdiich  these  essays  may  afford,  the  illustrations  may  fairly  be 
reckoned  as  the  element  of  most  importance  in  this  work, 

N.  S.  S. 


CONTENTS. 


THE    STABILITY    OF    THE    EARTH. 

PAGE 

Error  of  Ancient  View  concerning  Stability  of  tlie  Earth. — Steadfast  Growth  of  Conti- 
nents.— Cause  of  their  Growth  ;  Effects. — Classification  of  Movements  :  Swayings, 
Pulsations,  Tremors. — Volcanic  Movements. — Nature  of  Earthquake  Shocks  ;  Man- 
ner of  their  Movement  through  the  Rocks  ;  Ways  in  which  tliey  are  Produced. 
— Regional  Distribution  of  Earthquakes. — Effects  of  Earthquakes  on  Society. — 
Seismic  History  of  North  America. — Classification  of  Earthquake  Shocks  accord- 
ing to  Energy  of  Disturbance. — Evidence  as  to  Former  Energy  of  their  Shocks. — 
.  Methods  of  Construction  by  which  the  Effects  of  Shocks  on  Buildings  maybe 
•  .Diminished. —^ Ocean  Waves  Produced  by  Earthquakes;  their  Distribution. — 
Absence  of  such  Wave  Action  on  the  Atlantic  Coast  of  North  America. — Nature 
of  Evidence  as  to  Former  Action  of  Ocean  Waves  Produced  by  Earthquakes. — 
Summary. — Conclusion I 

VOLCANOES. 

Uniformity  of  Action  of  Earth's  Machinery. — Historj'  of  Vesuvius  ;  Period  of  Greek 
Settlements  ;  Earthquakes  of  A.  D.  63  ;  Eruption  of  A.  D.  79  ;  Story  of  the  Death 
of  Pliny  ;  Changes  Produced  by  this  Eruption  ;  Herculaneum  and  Pompeii  ; 
Eruptions  after  79. — Present  Condition  of  Vesuvius  ;  Observations  on  Eruption  of 
18S2  ;  Lessons  concerning  Volcanic  Eruptions  from  this  Eruption. — Other  Italian 
Volcanoes. — Icelandic  Volcanoes  ;  Eruption  of  Skaptar  in  1783  ;  Effect  on  Aspect 
of  Sky.  —  Volcanoes  about  Pacific  Ocean  :  Malayan  Volcanoes. —  Eruption  of 
Krakatoa  in  i883.^Cause  of  Volcanic  Eruptions  ;  Method  of  Inquiry. — Distribu- 
tion in  Space  and  Time. — Daubree's  Experiment. — Effect  of  Accumulation  of 
Strata, — Comparison  with  Blast  Furnace  ;  with  Natural  Gas  Wells. — Evidence  from 
.i^tna. — Comparison  of  Lunar  and  Terrestrial  Volcanoes. — Effects  of  Volcanic 
Action 46 

CAVERNS    AND    CAVERN    LIFE. 

Effect  of  Caverns  on  Imagination.  —  Classification  of  Caverns.  —  Limestone  Caves: 
Method  of  Formation. — Caverns  of  Kentucky. — Sink  Holes,  Shafts,  Domes,  and 
Galleries. — ^^Formation   of  Stalactites. — Natural  Bridges. — Air  of  Caverns  :  Effects 


xii  CONTENTS. 


PAGE 


on  Decay  ;  on  Health. — Relation  of  rrimeval  Man  to  Caverns  ;  Dwellings  ;  Burial- 
Piaces. — Remains  of  Animals. — Living  Animals  of  Caverns  ;  Bearing  of  Evidence 
on  Darwinian  Theory.— Geographical  Distribution  of  Limestone  Caverns. — Hot 
Spring  Caves  ;  Mineral  Deposits  in  such  Caverns. — Fault  Caverns. — Wave  Caverns  : 
Blue  Grotto  ;  Staffa  Cave. — Rock  House  Caves. — Lava  and  other  Volcanic  Caverns. 
— Symmes's  Hole 9^ 


RIVERS    AND    VALLEYS. 

Advantages  of  Beginning  Study  of  Geology  with  River  Action. — Description  of  a 
River  Valley. — History  of  Rain-Drops  ;  Mountain-Torrents  ;  Processes  of  Erosion. 
—Passage  from  Torrents  to  Rivers.— Alluvial  Terraces. — Elifect  of  Plants  on  Allu- 
vium.-Effect  of  Tributary  Streams.— Ox-bows  and  Moats. — Function  of  Alluvial 
Plains. — Effect  of  these  Deposits  on  Conditions  of  Ocean  ;  Erosion  of  River  Chan- 
nels.—Waterfalls.— Classification  of  Cataracts.— Niagara  Falls:  Effect  of  their 
•  Recession.— Effect  of  Elevation  of  Continents.— Base  Level  of  Erosion.— Action 
of  Subterranean  Water. — Wanderings  of  Rivers. — Distribution  of  Streams.  —  Effect 
of  Changes  of  Elevation  of  the  Land  on  Rivers. — Effect  of  Mountain  Systems. — 
Geological  Consequences  of  Distribution  of  Rivers. — Comparison  of  Ohio  with 
Colorado  River.— Formation  of  ^////t-j.— Deltas:  their  Influence  on  Man;  Advan- 
tages for  Primitive  Peoples. — Change  and  Destruction  of  River  Valleys. — Dis- 
locations ;  Glacial  Deposits  ;  Lava  Streams. — Evidence  from  Old  River  Channels 
Concerning  Antiquity  of  Men.— Effect  of  Forests  on  Rivers.— Problem  of  the 
Mississippi  River. — Control  of  Floods. — Danger  from  Reservoirs  in  River  Valleys. 
— Irrigation ^^+3 


THE    INSTABILITY    OF    THE    ATMOSPHERE. 

Contrast  between  Conditions  of  Ocean  and  Air  ;  Mingling  of  these  Elements  ;  Depend- 
ence of  Organic  Life  upon  them. — Maintenance  of  Temperature  ;  Conditions  of 
other  Bodies  in  Solar  System  ;  Conditions  of  Moon ;  Way  in  which  Heat  is 
Retained  ;  Conditions  of  Temperature  in  Past  Geologic  Ages.— Evidence  of  Long- 
Continued  Equilibrium  in  Atmosphere  both  in  Heat  and  Constituents.— Elimination 
of  Oxygen  and  Carbon  from  Air;  Methods  of  Replacement.— Circulation  of  the 
Air  ;  Cause  of  Movements  ;  Tropical  Winds  ;  Trade  Winds  ;  their  Permanence  ; 
their  Origin. — Direction  of  Movement  of  Trade  Winds  ;  Cause  of  Inclination  to 
the  Equator.— Effects  of  Irregular  Distribution  of  Heat  ;  Compensating  Influence 
of  Ocean  Currents.— Inconstant  Winds  ;  their  Origin  ;  Land  and  Sea  Breezes  ; 
East  Wind  of  Atlantic  Coast.— Variable  Winds  ;  their  Origin  ;  Experiments  ;  Cause 
of  Whirling  Movement  ;  Various  Conditions  of  Origin.— Tornadoes  ;  Origin  of  ; 
Effects  of.  — Speed  of  Movement  and  Appearance  of  Tornadoes  ;  Origin  of  Destruc- 
tive Action  ;  Width  of  Path;  Ways  of  Avoiding  Accidents.— Distribution  of  Torna- 
does ;  Effects  on  Forests.— Cyclones  ;  their  Origin  ;  How  Ships  may  Avoid  them.— 
Effects  of  Cyclones  on  Shore  ;  in  Bay  of  Bengal  ;  in  Florida.— General  Economy  of 
Atmospheric  Movements ^97 


CONTENTS.  xiii 


• « 


FORESTS    OF    NORTH    AMERICA".' 


PAGE 


Arboreal  Ancestry  of  Man  ;  Need  of  Destroying  the  Forests  ;  Evil  Effects  of  Deforest- 
ing ;  Relation  of  Forests  to  Soil  ;  Protective  Effects. — Origin  of  Forest  Trees  ; 
Geologic  Succession  of  Plants  ;  Forests  of  Coal  Measures  ;  Evolution  of  the  Form  ; 
Comparison  of  Broad  and  Narrow  Leaved  Trees. — Study  of  a  Forest  District  ;  Purity 
of  Forest  Streams  ;  Compared  with  those  of  Tilled  Districts.— ;So;l  of  Forest. — 
Forest  Sponge  Effect  on  Rainfall;  Instances  from  Appalachian  Forests. — Variety  of 
Trees  in  Forests  of  North  America  ;  Comparison  with  Europe  ;  Advantages  arising 
from  this  Variety. — Eald  Cypress  ;  its  Knees. — Sour  Gum  ;  its  Root  Eoops. — 
Willows. — Effect  of  Position  on  Trees.— Recovery  of  Land  by  Forests  ;  in  Southern 
States  ;  in  New  England. — Comparative  Vigor  of  Conifers  and  other  Trees. — Effects 
of  Glacial  Period  on  Forests;  Processes  of  Selection.— Origin  of  Prairies  ;  Effect 
of  Fires  ;  Reforesting  of  Prairies. — Underground  Work  of  Forests  ;  Effects  of 
Carbonic  Acid  Gas. — Economic  Value  of  Forests  ;  Effects  of  Deforesting  on  Ameri- 
can Rivers  ;  Remedial  Measures. — Present  Condition  of  American,F6rests 257 


THE    ORIGIN    AND    NATURE    OF    SOILS. 

Relation  of  Soils  to  Organic  Life.— Origin  of  Soils  ;  Effects  of  Solar  Heat  ;  Influ- 
ence of  Atmosphere. — How  to  Begin  the  Study  of  Soils. — Stages  of  Soil  Forma- 
tion ;  Action  of  Rain  ;  of  Frost.— Effect  of  Lichens  ;  of  Higher  Plants.— Study 
of  Mountain  District.— Effect  of  Joints  ;  of  Roots  of  Trees.— Processes  of  Torrent 
Valleys  ;  Floods  ;  Rock  Avalanches.— Diablerets  ;  Goldau  ;  Yvorgne  ;  White 
Mountains. — Alluvial  Plains  ;  Processes  of  Formation  of  ;  Soils  of. — Upland  Soils  ; 
Immediate  Derivation  of  ;  Structure  of ;  Processes  of  Formation  of. — Process  of 
Ablation.- Glacial  Soils  ;  Conditions  of  Continental  Glaciers  ;  Nature  of  Glacial 
Soils  ;  Origin  of  Fertility  of  Soils  ;  Phosphate  Matter  :  its  Origin.— Effects  of 
Penetrating  Air  ;  Means  whereby  Air  enters  the  Soil.— Effects  of  Tillage  ;  Action 
of  the  Plough.— Proportion  of  Plant  Food  in  Soil.— Effect  of  Ferments.— Compari- 
son of  Natural  and  Artificial  Conditions  of  Soil.— Working  of  Soils  under  Tillage. 
— Man's  Duty  by  the  Soil. — Effects  of  Irrigation 300 


LIST    OF    ILLUSTRATIONS. 


FULL    PAGE    ILLUSTRATIONS. 

Canon  of  the  Colorado,  Frontispiece. 

Facing;  Page 

Crack  in  the  Ground,  formed  by  the  Charleston  Earthquake,        .  36 

Four  Stages  of  a  Volcanic  District,            62 

Crater  in  the  Sandwich  Islands  at  the  End  of  an  Eruption,         .  92 
Rent  in  the  Earth  from  which  Sulphurous  Vapors  attendant  on" 

an  Eruption  have  escaped 94 

Entrance  to  the  Mammoth  Cave,  Kentucky, 98 

Natural  Bridge,  Virginia, 114 

Brand's  Cascade,  Luray  Cavern, 122 

Rape's  Chasm,  near  Gloucester,  Massachusetts,          .        .        .        .  130 

Stream  Bed  with  Bowlders, 158 

Canon  of  the  Via  Mala,  Switzerland, 174 

Gorge  and  Fall  of  the  Yellowstone  River, 188 

Filling's  Cascade,  Utah, 194 

Instantaneous  Views  of  a  Tornado, 242 

Grades  of  Destruction  from  Centre  to  Periphery  of  a  Tornado,  250 

A  Tulip  Tree,  Bell  County,  Kentucky,            272 


ILLUSTRATIONS    IN    THE    TEXT. 

Page 

Section  through  Mont  Blanc,  Switzerland 3 

Diagram  showing  the  Geological  History  of  the  Temple  of  Jupiter 

Serapis, 8 

Dyke  of  Volcanic  Rock i6 

Pinnacled  Rocks 21 

Erosion  Ojlumn 24 

Pinnacled  Rock  at  Cumberland  Gap 25 

Steepled  Rocks  and  Erosion  Bowlder, 28 

Erosion  Arch •  3° 

Spring-hole, 34 

"  Navajo  Church  "  Pinnacled  Rock 37 

Street  in  Charleston 40 

Effect  of  a  Powerful  Earthquake  on  Massive  Masonry,  Italy,  .  43 
Diagrammatic  Sections  through  Mount  Vesuvius,  showing  Changes 

in  the  Form  of  the  Cone,      .        .        .        . 47 

Diagrammatic  Section  through  Vesuvius,  in  Time  of  Eruption,     .  49 

View  in  Pompeii,  looking  Northwest, 52 

View  of  Excavated  Portion  of  Pompeii,  L()f)KiNG  Northwest,          .  55 

A  Lava-stream  Overwhelming  a  Town  on  the  West  Side  of  Vesuvius,  58 

Vesuvius  ;   Near  View  of  the  Small  Inner  Cone  of  the  Crater,      .  61 

Vesuvius,  looking  East  from  the  "  Oisservatory,"     ....  65 

Volcanic  Tufa  of  Naples 67 

Crater,  Lakes  of  the  Seven  Cities,  St.  Michael's,  Azores,        .        .  69 
Volcanic  Cone,  Sandwich  Islands,     .        .        ...        .        .        .73 

Crater  in  the  Sandwich  Islands  at  the  Close  of  Eruption,          .  76 

Lake  of  Lava  in  the  Sandwich  Islands, 79 

Border  of  Lava-stream  in  the  Sandwich  Islands 83 


ILLUSTRATIONS  IN  THE  TEXT.  xvii 

Page 

Hypothetical  Section  through   Rocks  near  a  Fault  on  which  a 

Line  of  Volcanoes  has  Formed, 87 

Front  of  a  Lava-stream  Falling  in  Rivulets  into  the  Sea,  Sand- 
wich Islands, go 

The  Same  Lava-stream  Pouring  in  Full  Tide  into  the  Sea,      .        .  93 
Lava-stream  at  Point  of  Egress,  with   Escaping  Steam,   Sandwich 

Islands, ^6 

Cave  Hill,  with  Sink-holes,  Luray,  Virginia, loi 

Sink-holes,  Edmonson  County,  Kentucky, 103 

Sink-hole,  Edmonson  County,  Kentucky, 104 

Stalactites,  Luray  Cavern 107 

Formations  of  Caverns  in  Limestone, 109 

Stalactite  Formation  in  Limestone, no 

A  Stalactite.    Cross-section  of  Stalactite, 112 

Stalactites,  Luray  Cavern, 113 

Sea-Shore  Cave, 130 

The  Blue  Grotto,  Island  of  Capri, 134 

Cave-dwellings,  Nevada, 137 

Cave  under  Lava  Crust,  Sandwich  Islands, 138 

Torrent  Bed  in  Eastern  Kentucky 147 

Valley  showing  the  Beginning  of  New  Terraces 149 

Border  of  Alluvial  Terrace  on  Green  River,  Kentucky,       .        .  151 
Cl^mberland  River,  Kentucky,  from  Taylor's  Hill,      .        .        .        .153 

Alluvial  Terraces  of  Soft  Material  Rapidly  Eroded  by  a  River,  157 

Diagram  of  Waterfall  of  Niagara  Type, 162 

Cascade  de  la  Sirena,  near  La  Guayra,  Venezuela,          .        .        .  164 
Diagram  showing  Successive  Stages  of  Erosion  in  a  Valley  Under- 
laid BY  Tilted  Rocks  of  Varying  Hardness,          .        .        .        .167 
Diagram  showing  Gravel  Terraces,  each  marking  a  Stage  of  Down- 
cutting  BY  A  River,           171 

Dunkar  Spiti,  India 178 

View  into  a  Mountain  Gorge, 179 

Norwegian  Fjord, 181 


xviii  ILLUSTRATIONS  IN  THE  TEXT. 

Page 

Diagram  showing  Old  River  Channels  on  Top  of  Hills,         .        .  182 

Volcanic  Necks  in  the  Valley  of  the  Puerco, 183 

Land  and  Sea  Breezes,  No.  i.  Currents  of  Air  by  Day,           .        ,  225 

Land  and  Sea  Breezes,  No.  2.  Currents  of  Air  by  Night,           .        .  225 

A  Whirlwind 226 

Diagram  of  a  Sink-spout, 230 

A  Dust-whirl, 231 

Smoke-whirl  from  Forest  Fires, 232 

A  Water-spout, 234 

Section  through  a  Tornado, 235 

Effect  on  a  Train  in  the  Centre  of  a  Tornado 238 

Explosive   Effect   of   Air   contained    in   the   Hollow    Wall   of   a 

Building, 240 

Narrow  Limits  of  the  Destruction  and  Completeness  of  the  Ruin 

WITHIN  THE  Limited  Field, 241 

The  Overturning  Action  of  a  Tornado  on  Buildings,        .        .        .  244 

Grades  of  Destruction  from  Centre  to  Border  of  a  Tornado,    .  246 

Sharp  Passage  from  Centre  to  Periphery  of  a  Tornado,           .        .  248 
Overturned  Train  ;   showing  Effects  at  Some  Distance  from  the 

Centre  of  a  Tornado, 252 

Effects  on  a  Train  close  to  the  Centre  of  a  Tornado,            .        .  256 

Cycad  in  the  Botanical  Gardens,  Cape  Town,  South  Africa,         .  263 

A  Group  of  Palms,  Florida, 266 

Stream  obstructed  by  Fallen  Timber, 269 

Black-walnut,  Floyd  County,  Kentucky 270 

Yellow-pine,  Harlan  County,  Kentucky 271 

Swamp-cypress  (Tennessee),  showing  the  Spurs 277 

Black-jack  Oaks,  Todd  County,  Kentucky, 280 

Winged  Elm  (showing  Foliage  on  the  Edge  of  a  Forest),  Cumber- 
land Valley,  Kentucky, 284 

Sycamore  Tree  in  White  River  Bottoms,  near  Wheatland,  Mary- 
land,        287 

Ash  Grove,  Ashland,  Fayette  County,  Kentucky,        ....  2S9 


ILLUSTRATIONS  IN  THE  TEXT.  xix 

Page 

Diagram  showing  Conditions  of  Torrent  Valley  in  which  Alluvial 

Soils  are  forming 304 

Diagram  showing  Conditions  of  a  Great  Landslide,  .        .        .     310 

View  of  Stream  at  Part  where  it  passes  from  Torrent  Section  to 

River,  Eastern  Kentucky 314 

Diagram  Showing  Portion  of  Alluvial  Terraces  in  a  River  Val- 
ley,         316 

Section  through  Forest  Mould  Soil  and  Sub-soil,  showing  Action 

of  Roots, 324 


ASPECTS  OF  THE  EARTH. 


THE    STABILITY    OF    THE    EARTH. 

Error  of  Ancient  View  concerning  Stability  of  the  Earth. — Steadfast  Growth  of  Continents. 
— Cause  of  their  Growth  ;  Effects. — Classification  of  Movements  :  Swayings,  Pulsations, 
Tremors. — Volcanic  Movements. — Nature  of  Earthquake  Shocks  ;  Manner  of  their 
Movement  through  the  Rocks  ;  Ways  in  which  they  are  Produced. — Regional  Distri- 
bution of  Earthquakes. — Effects  of  Earthquakes  on  Society. — Seismic  Historj'  of  North 
America. — Classification  of  Earthquake  Shocks  according  to  Energy  of  Disturbance. — 
Evidence  as  to  Former  Energy  of  their  Shocks.  —  .Methods  of  Construction  hy  which  the 
Effects  of  Shocks  on  Buildings  may  be  Diminished. — Ocean  Waves  Produced  by  Earth- 
quakes ;  their  Distribution. — Absence  of  such  Wave  Action  on  the  Atlantic  Coast  of 
North  America. — Nature  of  Evidence  as  to  Former  .\ction  of  Ocean  Waves  Produced 
by  Earthquakes. — Summary. — Conclusion.  — 

Human  society  is  organized  for  a  stable  earth  ;  its  whole 
machinery  supposes  that,  while  the  other  familiar  elements 
of  air  and  water  are  fluctuating  and  trustworthy,  the  earth 
affords  a  foundation  which  is  firm.  Now  and  then  this 
implied  compact  with  nature  is  broken,  and  the  ground  trem- 
bles beneath  our  feet.  At  such  times  we  feel  a  painful  sense 
of  shipwrecked  confidence  ;  we  learn  how  very  precious  to  us 
was  that  trust  in  the  earth  which  we  gave  without  question. 
If  the  disturbance  be  of  a  momentary  and  unimportant  kind, 
we    may  soon    forget    it,  as  we  forget   the    rash  word   of   a 


2  ASPECTS   OF  THE  EARTH. 

friend;    if  it  be  violent,  \vc  lose  one  of  the  substantial  goods 
of    life-,   our    instinctive  confidence  in   the  earth  beneath  our 

feet. 

The  notion  that  the  ground  is  naturally  steadfast  is  an 
error  -an  error  which  arises  from  the  incapacity  of  our  senses 
to  appreciate  any  but  the  most  palpable  and,  at  the  same  time, 
most  exceptional  of  its  movements.  The  idea  of  terra  tirma 
belongs  with  the  ancient  belief  that  the  earth  was  the  centre 
of  the  universe.  It  is,  indeed,  by  their  mobility  that  the 
continents  survive  the  unceasing  assaults  of  the  ocean  waves, 
and  the  continuous  down-wearing  which  the  rivers  and  glaciers 
bring  about. 

Were  it  not  that  the  continents  grow  upward,  from  age 
to  age.  at  a  rate  which  compensates  for  their  erosion,  there 
would  bt;  no  lands  tit  for  a  theatre  of  life  ;  if  they  had  grown 
too  slowly,  their  natural  enemies,  the  weaves  and  rain,  would 
have  kept  them  to  the  ocean  level  ;  if  too  fast,  they  would 
lift  thirir  surfaces  into  the  regions  of  eternal  cold.  As  it  is, 
the  development  has  been  so  well  measured  to  the  needs, 
that  for  a  hundred  million  years,  more  or  less,  the  lands 
have  afforded  the  stage  for  jjrosperous  life.  This  uprising, 
when  me.isurcd  in  t(;rms  of  human  experience,  is  slow  ;  it 
probably  doe-s  not  e.xceetl.  on  the  average,  one  foot  in  three 
_or  four  thousand  years.  Ilu'  rate;  varies  in  times  and  places. 
Under  varying  conditions,  as  when  a  glacial  sheet  is  imposed 
on  the  continent  ;is  ii  was.  in  the  immediate  past,  on  the 
northern  i)art  of  North  .America— a  wide  area  of  the  ice-laden 
land  sank  beneath  th<-  sea.  to  rec(n(M'  its  level  when  the 
depressing  bvirden  was  removed.  Still  the  tendency  of  the 
continents  is  to  elevation,  and  even  the  temporary  sinking  of 
one  portion  of  their  area  is  prob.ibly,  in  all  cases,  compensated 


THE   STABILITY  OF   THE  EARTH.  3 

by  uplifts  on  another  part  by  which   new  realms  of  land  are 
won  from  the  sea. 

Although  access  to  the  deeper  earth  is  denied  us,  we  are 
probably  safe  in  our  belief  that  this  steadfast  upward  move- 
ment of  the  lands  is,  in  the  main,  due  to  a  simple  cause,  which 
is  as  follows,  viz,  :  The  diameter  of  the  earth  depends,  in  part, 
upon  the  amount  of  heat  it  contains.  This  heat  is  constantly 
flying  out  into  space.  Each  moment,  from  every  part  of  its 
surface,  some  portion  of  the  original  store  escapes  into  the 
cold  realms  of  space.  With  every  volcanic  eruption  a  great 
outrush  of  heat  occurs.      Thus,  the  earth  is  steadfastly  shrink- 


Section  through  Mont  Blanc,  Switzerland. 
Showing  folds  of  strata  in  a  mountain. 

ing  :  each  age  it  is  girdled  by  a  shorter  line.  If,  by  this  escape 
of  heat,  every  part  of  the  earth  were  equally  cooled,  there 
would  be  no  continents,  for  the  whole  mass  would  fall  equally 
toward  the  centre  ;  but  the  deeper  parts  of  the  earth  lose  by 
far  the  most  heat,  for  the  simple  reason  that  they  have  the 
most  to  part  with.  The  superficial  portions  long  since  parted 
with  the  larger  part  of  their  original  caloric. 

Thus,  this  upper  portion,  or  crust,  as  it  is  commonly  called, 
does  not  contract  as  much  as  the  interior  mass,  and  therefore 
the  inner  part  tends  to  leave  the  outer  crust  behind.  But  for 
the  weight  of  this  outer  section,  it  would  be  left  more  or  less 
separated  from  the  interior  mass  ;  but,  as  its  weight  is  much 
greater  than   it  can  sustain,  it  is  compelled  to  wrinkle,  or,  in 


4  ASPECTS   OF  THE  EARTH. 

Other  words,  to  form  the  great  ridges  and  furrows  which  con- 
stitute the  continents  and  the  ocean  basins.  Geologists  are 
still  in  debate  as  to  the  precise  manner  in  which  this  wrinkling 
comes  about,  and  as  to  the  way  in  which  it  has  effected  the 
construction  of  continents  and  mountains ;  but  they  very 
generally  believe  that  it  is  principally  due  to  the  cause  above 
mentioned — /.  e.,  to  the  loss  of  heat,  which  is  greater  from  the 
interior  than  from  the  superficial  parts  of  the  earth.  In  a 
rough  way,  this  folding  of  the  outer  part  of  the  earth  may  be 
compared  to  the  wrinkling  of  the  skin  of  a  dried  apple  ;  only 
in  the  fruit  the  shrinkage  of  the  interior  is  due  to  the  escape 
of  water,  while  in  the  case  of  the  earth  it  is  due  to  the  loss 
of  energy  in  the  form  of  heat. 

It  is  easy  for  the  reader  to  see  that  this  wrinkling  sets 
a  vast  amount  of  machinery  in  operation,  and  compels  the 
movement  of  masses  which  cannot  be  expected  to  stir  without 
shock.  In  the  upward  folding  of  continents  and  of  moun- 
tains, the  rocks  must  bend  and  break,  oreat  fragments  of  rock 
must  slide  over  each  other,  making  such  flexures  as  are  seen 
on  existing  mountains  or  in  regions  where  mountains  have 
once  lifted  their  ridges,  though  they  may  now  be  worn  down 
to  their  roots,  and  no  longer  have  any  trace  of  their  original 
altitude.  This  folding  is  titanic  work,  and  the  movements  at 
great  depths  beneath  the  surface  made  necessary  by  this 
wrinkling  demand  very  extensive  disturbances  of  vast  masses 
of  the  earth  ;  these  uplifted  arches  of  the  mountains  have  to 
be  underpinned,  or  supported  from  below,  else  they  would 
crush  down  by  their  own  weight  ;  this  support  from  beneath 
demands  the  transfer  from  considerable  distances  within  the 
crust  on  either  side  of  the  mountain  of  large  quantities  of  rocky 
matter.     Although  this  rock  is  greatly  heated,  it  is  probably 


THE   STAB  I  LIT!'  OF   THE  EARTH.  5 

not.  in  a  strict  sense,  duid.  and  so  moves  with  a  certain  dilti- 
culty.  and  only  under  the  compulsion  of  inconceivably  crreat 
strains  which  cannot  be  expected  to  act  without  a  certain 
measure  of  disturbance.  Thus,  bv  the  folding,  breakino-,  and 
slipping  involved  in  the  production  of  the  greater  reliefs  of 
the  earth,  a  certain  amount  of  sudden  and  irregular  motion 
is  necessarily  brought  about. 

Beneath  the  sea  and  along  the  shores  we  have  another  dis- 
turbing agent  in  the  volcanic  impulse.  On  the  sea-floors  the 
mountain  and  continent-building  forces  appear  in  the  main  to 
be  wanting,  while  the  volcanic  conditions  are  at  rest  beneath 
the  interior  of  the  continents.  The  conditions  producing 
volcanoes  appear  to  originate  in  the  following  manner  :  The 
deposits  of  sedimentary  matter  which  are  constantly  making 
in  the  sea-floors  contain  a  great  deal  of  water  ;  from  five  to 
twenty  per  cent,  of  their  mass  consists  of  the  fluid  which  is 
imprisoned  between  the  grains  of  mud  or  sand  as  the  beds 
are  formed.  When,  in  time,  any  of  these  beds  become  deeplv 
buried,  they  become  greatly  heated  by  the  heat  of  the  earth's 
interior,  the  exit  of  which  is  hindered  by  the  strata  laid  down 
after  the  lower  beds  were  formed.  When,  in  this  way,  a  bed 
is  buried  to  the  depth  of  twenty  thousand  feet  or  more,  the 
imprisoned  water  may  be  raised  to  a  temperature  far  above 
its  ordinary  boiling  point.  Into  this  region  of  deeply  buried 
water-charged  beds  the  heat  comes,  not  only  by  conduction 
from  the  earth's  interior,  but  also  by  the  action  of  streams  of 
molten  rock,  which  rush  upward  from  below,  forming  dykes 
or  veins  of  lava,  such  as  may  often  be  seen  when  ancient  and 
once  deeply  buried  strata  are  disclosed  to  view  by  the  wearing 
away  of  the  deposits  which  formerly  lay  upon  them. 

This    <:reatlv    heated    water    of    the    rocks    is    constanth- 


6  ASPECTS   OF  THE  EARTH. 

seeking  to  pass  into  the  state  of  vapor;  if  it  finds  any  line 
of  weakness,  it  rends  it  open,  with  more  than  the  energy  of 
exploding  gunpowder,  and  forms  a  volcano.  Volcanoes  are 
essentially  gigantic  explosions,  such  as  are  faintly  imitated  in 
burstino-  steam-boilers.  In  the  volcanic  explosion  the  steam 
is  so  hot  that  it  may  melt  the  rocks  through  which  it  passes, 
or  drive  those  beds  in  which  it  was  formed  upward  to  the 
surface  in  the  form  of  lavas  or  finely  divided  dust. 

Thus  in  the  up-growing  of  the  lands  to  replace  the  con- 
tinued down-wearing  which  assails  them,  and  in  the  outbreaks 
of  the  heated  water  deeply  buried  in  the  sediments  derived 
fron-i  these  worn  lands,  we  have  two  evident  sources  of  earth- 
quake movements.  These  disturbances  express  themselves 
on  the  surface  simply  as  movements,  with  no  distinct  evidence 
as  to  the  origin  of  the  shock  ;  just  as,  when  we  hear  a  loud 
noise,  we  may  find  it  to  be  due  to  any  one  of  many  causes — 
to  a  falling  meteor,  the  firing  of  a  cannon,  a  bursting  boiler,  or 
something  else  in  the  way  of  sound-producing  action — so  with 
these  earthcjuake  shocks  :  they  tell  us  little  of  their  causation  ; 
that  is  the  subject  for  troublesome  and  often  bafifling  inquiry. 
Leaving  aside  the  great  slow  movements  of  the  lands  which 
we  cannot  feel,  and  can  only  infer  from  geological  monuments, 
such  as  ancient  shore-lines  or  the  marine  fossils  in  the  rocks 
w^hich  compose  high  mountains,  and  considering  only  the  sen- 
sible movements  of  the  earth's  crust,  we  find  several  distinct 
classes  of  motions  by  which  the  earth  is  affected.  Arranging 
these  in  the  order  of  their  magnitude  and  the  time  occupied 
b\-  their  movements.  w(;  have  tin;  groups  noted  below. 

r^irst  among  these  oscillations  of  the  earth  we  may  notice 
the  slow  up  or  down  movements,  which  are  probably  of  the 
same  <'-ener:d  nature  and  of  \\\r.  same  origin  as  the  movements 


THE   STABILITF  OF   THE  EARTH  7 

which  build  the  continents,  only  much  more  rapid  ;  so  rapid, 
indeed,  that  they  may  be  observed  from  decade  to  decade,  or, 
at  least,  from  century  to  century.  In  this  class  we  include  the 
down-sinking  of  the  coast  of  New  Jersey,  the  uprising  of  the 
northern  part  of  Scandinavia,  or  the  oscillations  of  the  shore 
on  the  coast  of  the  Bay  of  Naples.  These  movements,  which, 
though  in  a  geological  sense  rapid,  rarely  change  the  level  of 
the  land  more  than  a  foot  or  two  in  a  century,  appear  to  be 
divided  into  three  distinct  classes  as  follows :  First,  those 
which  are  due  to  the  imposition  of  a  heavy  weight  upon  the 
earth's  surface,  or  to  the  removal  of  such  a  weight,  A  good 
case  of  this  is  the  deep  depression  of  the  northern  part  of 
North  America  where  the  glacial  sheet  came  upon  it,  and  its 
rapid  reelevation  when  the  ice  melted  away.  Next,  those 
which  are  due  to  the  formation  of  a  great  fault  or  break 
through  the  rocks  as  they  are  shoved  about  by  the  compress- 
ive forces  which  build  mountain  chains.  And,  finally,  those 
which  are  due  to  the  movements  of  volcanic  erases  and  the 
lava  which  they  propel  toward  the  crater,  whence,  in  time, 
they  are  to  be  discharged. 

Of  these  slow  movements  the  most  interesting,  because 
the  best  known,  is  that  which  is  shown  by  the  ruins  of  the 
temple  of  Jupiter  Serapis,  near  Naples.  We  see  by  the  evi- 
dence of  these  ruins  that  the  temple  has  sunk  down  since  the 
Christian  era,  so  that  the  marine  animals  bored  into  the  mar- 
ble columns  at  the  height  of  more  than  twenty  feet  above  the 
present  level  of  the  sea  ;  it  then  rose  up  to  its  original  level, 
and  is  now  aofain  sinkinor  at  the  rate  of  one  inch  in  three  or 
four  years.  A  similar  movement  connected  with  the  process 
of  mountain-building  has  been  observed  at  Subiaco,  about 
forty  miles  to  the  north  of  Rome.      A  hundred  years  or  so  ago 


8 


ASPECTS  OF  THE  EARTH. 


the  church  of  Jenne  was  invisible  from  Subiaco,  while  now  it 
is  in  plain  view  over  the  summit  of  the  intervening  mountain. 
This  change  can  only  be  explained  by  an  alteration  in  the 
heio-ht  of  the  mountain  arches  of  this  district. 


Diagram  showing  the  Geological  History  of  the  Temple  of  Jupiter  Serapis.* 

Along  the  eastern   coast  of  the   United  States   there  are 
similar  gradual  changes  in  the  altitude  of  the  lands  in  relation 

*The  first  figure  shows  the  original  position  of  the  temple  ;  the  second,  the  con- 
<lition  at  the  time  of  the  greatest  submergence  ;  the  third,  the  present  position  of  the 
ruins.  The  reader  should  observe  the  changes  in  the  background  of  these  three  dia- 
grams. Although  not  intended  to  depict  the  details  of  scenerv  in  an  accurate  way, 
a  task  which  would  be  impossible,  they  serve  to  show  something  of  the  history  of 
the  region.  Thus,  'u\  the  uppermost  figure,  which  represents  the  temple  when  it 
was  perfect,  the  background  indicates  no  distinct  volcanoes.  Tiie  second  diagram 
is  unsatisfactory,  for  the  reason  that  the  shores  are  shown  as  higher  than  in  the  first 
figure,  while  in  fact  they  should  be  lower.  In  the  third  picture,  the  cone  on  the  left 
hand  of  the  figure  indicates  the  volcanic  hill  known  as  Monte  Nuovo,  which,  as  will 
be  seen  in  the  text  of  the  ch;i|)ter  on  volcanoes,  was  thrown  up  in  comparatively 
modern  times. 


THE   STABILITV  OF   THE  EARTH.  9 

to  the  sea.  Thus  along  the  New  England  coast  between 
New  York  and  Maine,  and  again  along  the  Gulf  of  St.  Law- 
rence, we  find  numerous  submerged  forests  with  quantities  of 
trees  standing  upright,  with  their  roots  in  old  forest  beds,  but 
with  their  crowns  some  feet  below  the  level  of  hieh  tide.  On 
the  coast  of  New  Jersey,  and  thence  southward  to  Florida, 
similar  evidence  indicates  that  the  marginal  part  of  the  conti- 
nent at  least,  and  perhaps  a  great  portion  of  its  interior  area, 
has  gradually  sunk  down  until  extensive  areas  recently  land 
are  now  submarine.  North  of  New  York  the  continent,  at 
least  along  the  shore  line,  appears  at  present  to  be  undergoing 
slight  changes  or  to  be  essentially  stable  ;  but  in  New  Jersey 
and  along  the  coast  to  the  southward,  there  are  reasons  to 
believe  the  down-sinking  continues  to  the  present  day.  It  is 
the  opinion  of  those  who  have  studied  the  facts  in  New  Jer- 
sey that  the  subsidence  is  now  going  on,  probably  at  the  rate 
of  two  feet  in  a  hundred  years,  the  effect  being  on  this  level 
coast  to  bring  the  sea  inward  with  considerable  rapidity  over 
the  lowlands,  thus  limiting  the  area  accessible  to  tillage  ;  at  the 
same  time  this  movement  tends  to  preserve  the  deep  water  in 
the  harbors  by  compensating  for  the  destruction  of  the  depth 
that  comes  about  through  the  deposition  of  alluvium  on  their 
floors. 

Another  class  of  earth  movements  includes  what  have 
been  called  by  Professor  Milne  earth  pulsations.  These  are 
temporary  slight  swayings  of  the  earth,  which,  though  occupy- 
ing a  short  time,  from  a  few  seconds  to  a  few  hours,  are  still 
so  slow  that  they  do  not  give  any  sense  of  shock.  These 
swayings  of  the  earth  have  been  best  observed  by  means  of 
delicate  spirit-levels,  the  bubbles  of  which  move  with  very 
slight  changes  of  level  at  either  end  of  the  instrument.      So 


,0  ASPECTS    OF  THE  EARTH. 

far  these  observations  have  been  made  at  but  few  points  and 
for  short  periods  of  time  ;  they  serve,  however,  to  show  that 
the  surface  of  the  earth  is  very  orenerally  subject  to  shght 
oscillations,  which  probably  depend  in  part  upon  changes  in 
the  weight  of  the  atmosphere  ;  they  may,  however,  in  part,  be 
due  to  the  varying  strains  which  are  produced  by  the  conti- 
nent- and  mountain-building  process,  and  perhaps,  in  certain 
regions  near  the   shore,  by   the  action  of  the  volcanic  forces 

as  well. 

Another  class   of   movements   has    received   the    name    of 
earth  tremors.      These  are  very  slight  jarrings  of  the  earth, 
too  trifling  to  make  any  impression  on  the  unaided  senses  ;  in 
fact,  only  made  sensible  by  means  of  very  delicate  pendulums 
and  other  contrivances  of  that  nature.     Whenever  such  obser- 
vations have  been  carefully  undertaken,  it  has  been  found  that 
the  surface  of  the  earth  is  in  a  state  of  recurrent  or  continu- 
ous   movement.      In  Italy,  where  these   inquiries   have   been 
most  continually  and  skilfully  made — where,  indeed,  this  branch 
of  geologic  study  originated — these  tremors,   though  observ- 
able  at   nearly  all   times,  are  characterized  by  fluctuations   in 
their  frequency  and  intensity.      During  a  time  of  great  baro- 
metric disturbance  the  oscillations  of  the  pendulum  are  often 
very  marked.      It  seems  certain    that  a  cause  apparently  as 
slight  as  the  sudden  changes  in  the  weight  of  the  atmosphere 
in  the  tumult  of  a  g-ale  is  sufficient  to  cause  the  elastic  crust 
of  the  earth  to  tremble.     Again,  in  the  days  preceding  a  sen- 
sible earth(|uake,  especially  one  of  any  violence,   the  instru- 
ments show  a  great  increase  in  the  trembling  movement.      It 
appears,  indeed,  as  if  in  time  we  may  be  able  to  foretell  the 
occurrence  of   important   shocks   by   their  forerunners   in   the 
shape  of  microscopic  movements  of  the  earth's  crust. 


THE   STABILITY  OF   THE  EARTH.  II 

With  the  microphone,  that  microscope  of  the  ear,  it  has 
been  shown  that  in  Italy,  and  probably  wherever  these  little 
earthquake  waves  occur,  the  earth  sends  forth  a  medley  of 
confused  sounds- — crackings  and  snappings — probably  caused 
by  the  rocks  creeping  toward  relief  from  the  strains  which 
urf^e  them  to  change  their  position.  It  is  hardly  too  much  to 
say  that  this  method  of  observing  the  earth  has  enabled  us  in 
part  to  perceive  the  constant  working  of  the  great  telluric 
machinery  which  continually  builds  our  lands. 

Between  the  class  of  earth  tremors  and  earthquakes  proper 
there  is  no  other  difference,  save  in  the  violence  of  the  shock. 
As  long  as  the  movements  are  imperceptible  to  the  unaided 
human  senses,  we,  for  convenience,  place  them  in  the  former 
group  ;  when  they  are  great  enough  to  excite  our  senses  they 
are  called  earthquakes. 

What  has  been  already  said  has  probably  made  it  clear 
to  the  reader  that  an  earthquake  shock,  like  any  other  jar,  is 
only  the  result  of  some  disturbance,  and  not  in  itself  an 
original  fact.  In  order  fully  to  understand  what  happens  in 
any  shock,  it  is  necessary  to  look  a  little  more  closely  at  the 
nature  of  these  vibrations  of  the  rocks  which  constitute  earth- 
quakes. The  ordinary  experiences  of  life  make  us  in  many 
ways  familiar  with  the  elasticity  of  common  substances ;  a 
boy's  marble,  which  is  composed  either  of  compact  limestone 
or  of  glass,  though  not  evidently  elastic  to  a  pressure  of  the 
fingers,  will  bounce  like  a  rubber  ball  if  thrown  upon  a  pave- 
ment of  hard  stone.  A  bullet  fired  against  a  stone  wall  will, 
as  many  a  boy  has  noticed,  be  hurled  back  with  menacing 
velocity.  Such  facts  show  us  that  if  rocks  are  struck  a  blow 
of  sufficient  violence,  they  act  as  very  elastic  substances. 
Further   experiments,    also    familiar   in    the   arts,   extend   this 


,2  ASPECTS   OF  THE  EARTH. 

conception.  When  a  strong  blast  of  gunpowder  or  other 
similar  explosive  is  fired,  as  in  quarries  or  mines,  the  shock 
extends  for  great  distances.  Thus,  in  the  last  great  explosion 
in  the  mines  used  for  the  destruction  of  the  reefs  at  Hell 
Gate,  near  New  York,  the  shock  was  distinctly  perceptible  at 
a  distance  of  more  than  one  hundred  miles  from  the  point 
where  the  blow  was  struck,  and  was  possibly  evident  nearly 
two  hundred  miles  away. 

Returning  to  the  instructive  experiment  with  the  marble, 
we  observe  that  its  elasticity  is  not  manifested  by  a  single 
bounce,  but  that  it  again  and  again  rebounds  from  the  stone 
floor  before  it  comes  to  rest.  This  is  because  gravity  impels 
it  downward  after  each  blow  sends  it  up  ;  but  if  w^e  could 
suspend  the  ball  in  air  after  its  first  rebound,  we  should  see 
that  the  little  sphere  vibrated  for  some  time  after  the  blow  ; 
we  should,  under  favorable  circumstances,  see  that  it  simply 
changed  its  form  from  a  sphere  to  a  spheroidal  shape  in  a 
regular  pulsating  way.  If  we  observe  the  rocks  near  a  blast 
discharged  in  a  quarry  or  mine,  our  instruments  show  us  that 
the  rock  of  the  earth's  crust  vibrates  in  essentially  the  same 
manner  as  the  marble — it  swings  to  and  fro  until  the  force 
which  set  it  in  motion  is  exhausted  in  the  frictions  which  the 
impulse  encounters. 

One  more  peculiar  experiment  will  enable  the  reader  to 
complete  his  conception  of  the  important  features  of  earth- 
quake waves,  It  is  easy  to  remember  what  happens  if  w^e  jar 
the  centre  of  a  still  basin  of  water,  as  we  do  when  we  apply 
a  certain  amount  of  forc<'  to  it  by  tossing  a  pebble  upon 
its  surface — wavelets  are  formed,  which  roll  awa)'  from  the 
centre.  \\'e  can  easily  see  that  these  wavelets  are  mere 
wrinkles    in    the    water,    created    like    thos(!    we   may    form    in 


THE   STABILJTF  OF  THE   EARTH.  13 

a  Strip  of  carpet  when  we  shake  it  on  the  floor.  If  in  their 
on-going  these  water-waves  strike  against  any  body  which 
does  not  move  with  them — a  floating  cake  of  ice,  for  instance 
— other  Httle  waves  start  back  from  the  resisting  objects, 
which  cross  and  mingle  with  the  original  waves  and  partly 
destroy  them.  It  is  now,  we  may  hope,  not  difficult  to  con- 
ceive that  the  waves  started  in  a  mass  of  rock,  which  are,  in 
a  certain  general  way,  like  the  undulations  of  the  water,  may 
move  in  any  direction  from  the  point  where  they  were  created 
by  the  jar,  until  they  come  to  the  earth's  surface,  or  are  worn 
out  by  the  frictions  which  they  have  to  overcome.  We  also 
can  conceive  that  diverse  accidents  in  the  rocks  may  much 
affect  the  movement  of  these  waves  of  elastic  compression,  as 
they  are  called,  which  constitute  an  earthquake.  When  the 
rock  is  much  rifted,  they  may  quickly  be  extinguished  ;  when 
it  is  spongy  and  inelastic,  they  may  rapidly  die  away.  It  is 
less  easy  to  see  that  when  a  vibration  is  running  through  rock 
of  a  given  elasticity,  and  encounters,  as  it  well  may,  a  kind  of 
rock  of  another  degree  of  elasticity,  it  will  have  its  waves 
reflected,  much  as  those  of  the  pool  of  water  are  reflected 
from  a  floating  cake  of  ice,  and  so  make  a  confusion  of  cross- 
vibrations,  which  may  very  much  vary  the  action  of  the  origi- 
nal movements. 

We  turn  now  to  consider  more  in  detail  the  causes  of 
earthquakes.  We  have  seen  that  the  power  which  urges  the 
continents  into  their  great  folds,  or  the  mountains  into  the 
lesser  corrugations,  affords  a  simple  and,  indeed,  necessary 
cause  of  certain  violent  strains,  which  in  their  action  tend  to 
compress  the  rocks  into  a  less  bulk  than  they  originally  occu- 
pied. Under  the  influence  of  these  strains,  any  one  or  more 
of  the  following  accidents  may  occur  :   The  rocks  may  wrinkle 


14  ASPECTS   OF   THE  EARTH. 

into  folds  like  those  we  find  in  mountains  ;  they  may  be 
broken  along  their  natural  joints  or  fracture-planes,  and  the 
sundered  parts  may  slip  over  each  other,  or  the  rocks  may 
be  squeezed  (uit  like  dough  under  the  cook's  roller,  and  so 
escape  to  a  region  of  less  pressure.  In  taking  any  of  these 
methods  of  relief,  the  rocks  are  necessarily  liable  to  many 
sudden  starts,  each  accompanied  by  rendings  and  other  violent 
movements. 

When,  in  winter,  the  frozen  ground  rives  asunder,  though 
the  crevice  is  but  a  small  fraction  of  an  inch  wide,  and  a  foot 
or  two  deep,  the  ground  is  often  so  violently  jarred  that  a 
sensible  earthquake  is  produced,  w^hich  may  be  felt  hundreds 
of  feet  away  froni  the  source  of  the  disturbance.  It  is, 
indeed,  likely  that  many  of  the  slight  local  earthquakes  which 
are  chronicled  may  be  due  to  this  cause.  We  can,  therefore, 
readily  see  that  when  the  fracture  has  a  length  of  miles,  and 
a  depth  of  thousands  of  feet,  as  is  the  case  in  many  faults, 
the  jar  occasioned  may  produce  a  disastrous  shock,  which  may 
involve  a  great  area  of  country.  But  it  is  not  only  the 
rending  of  the  fissures  which  produces  the  jar;  after  the  rift 
is  formed  the  severed  masses  of  rock  slip  over  each  other — 
the  rocks  on  one  side  rising,  while  those  on  the  other  side 
slip  down. 

Thtrsc  two  great  walls  on  either  side  of  the  fault  are  not 
smooth,  but  each  is  jagged  with  projections,  which  are  often 
rui)tur(Ml  as  the\-  grind  against  each  other  in  their  opposed 
movement.  Hv  each  of  these  minor  rendings,  as  well  as  by 
the  formation  of  th(^  j)rincipal  fracture,  the  rocks  are  set 
a-quivering  like  the  bounced  marble  ot  our  |)revious  illustra- 
tion. Th<-s«'  faults  are,  indeed,  earth(]uake  factories.  The 
greatest  shock  is  prcMluced  at  th<'  time  of  their  formation  ;  but 


THE   STABILITF  OF   THE  EARTH.  1 5 

from  time  to  time  they  are  freshly  ruptured,  perhaps  after  a 
vein  deposit  has  bound  their  adjacent  walls  together,  and  the 
disturbance  is  again  and  again  renewed.  It  is  evident  that 
many  great  faults — those  in  which  the  slipping  of  the  sides  on 
each  other  has  amounted  to  a  thousand  feet  or  more — have 
moved  only  a  few  inches  at  any  one  time,  so  that  a  single 
such  fracture  may  have  given  rise  to  hundreds,  if  not  thou- 
sands, of  earthquake  shocks.  So,  too,  when  beds  are  not 
broken,  but  are  bent  into  an  arch,  the  rocks  must  slip  over 
each  other.  The  reader  will  see  this  illustrated  if  he  bends 
the  hundred  pages  he  is  reading  into  a  sharp  fold.  Suppos- 
ing the  pages  were  from  the  great  stone-book  of  the  earth's 
crust,  and  the  thickness  of  each  leaf  many  feet,  and  of  the 
whole  many  tens  of  thousands  of  feet,  it  is  easy  to  conceive 
that  the  motion  would  not  take  place  silently,  but  with  much 
perturbation.  There  is  in  mountain-building  a  chance  for 
many  slight  shocks,  with  but  a  small  amount  of  motion.  In 
the  formation  of  such  folds  as  those  of  Mont  Blanc  the 
tremors  may  have  been  numbered  by  the  million. 

Another  most  evident  class  of  shocks  have  their  origin  in 
the  movement  of  rocks  which  have  been  melted  by  volcanic 
action,  and  driven  on  hurried  incursions  into  crevices  which 
are  formed  in  the  deep  buried  parts  of  the  earth's  crust.  To 
see  the  origin  of  these  disturbances,  we  have  only  to  visit  any 
of  those  regions  of  the  earth  once  deeply  covered  by  strata 
which  have  been  worn  away,  revealing  to  us  rocks  which,  by 
their  crystalline  structure,  indicate  their  long  sojourn  in  the 
depths  where  the  volcanic  forces  are  developed. 

We  find,  in  almost  any  region  where  these  crystalline 
rocks  are  well  exposed  to  view,  many  long  tongues  of  lava, 
which  have   been  violently  driven  into  fissures  of  the   rock, 


i6 


ASPECTS   OF  THE  EARTH. 


riving  them  with  destructive  power.  The  formation  of  such  a 
dyke-fissure  and  the  inrush  of  the  lava  must  have  occasioned 
a  very  great  jarring  of  the  earth's  surface  beneath  which  the 
movement  occurred.  If  the  reader  is  famihar  with  the  sea- 
shore, he  must  often  have  noticed,  in  times  of  storm,  the 
quiver  which   the  stroke   of  a  great  wave  gives  to   the  rocks 


Dyke  of  Volcanic   Rock. 

In  prnnitc  on  the  shore,  in  Marblehead,  Mass.     The  horizontal  plan  shows  many  small  faults  which  have 
been  formed  since  the  dyke  was  made,  each  giving?  rise  to  an  earthquake. 

when  it  rushes  into  some  crevice  or  chasm,  and  is  tossed  into 
spray  b)-  the  l)lo\v.  Tliis  phenomenon  will  help  him  to  fancy 
how  gHMt  must  be  the  disturbance  when  a  molten  lava,  three 
or  tour  times  as  heavy  as  v/ater,  is  driven  into  the  rocks, 
l>rrhaps  with  a  greater  impulse  than  that  which  propels  the 
ball  from  a  cannon. 

These  dykes  are,  like  the  faults,  inconceivably  numerous. 
All   the   evid(-nce  goes  to   show   that   ihey  commonly  exist   to 


THE  STABILITY  OF  THE  EARTH.  I  7 

the  number  of  hundreds  beneath  each  square  mile  of  the 
earth's  surface.  In  certain  places,  the  rocks  are  fairly  laced 
Avith  them. 

Leaving  out  of  account  the  minor  sources  of  disturbance 
which  come  from  the  tumult  of  volcanic  explosions,  and  the 
stresses  arising  from  the  change  in  the  volume  of  rocks  under- 
going alterations  in  chemical  composition,  and  from  loss  and 
gain  of  heat,  we  see  that  in  the  evident  mechanism  of  the 
earth  we  have  the  natural  source  of  innumerable  earthquake 
shocks.  It  is  almost  certain  that  at  one  time  or  another 
every  portion  of  the  earth's  surface  has  felt  these  disturb- 
ances ;  it  is  equally  clear  that  the  shocks  have  not  at  any  time 
been  equally  common  on  all  parts  of  the  earth's  surface,  for 
the  reason  that  the  machinery  which  produces  them  is  often 
dormant  for  long  periods  over  large  areas.  A  mountain 
system,  after  continuing  to  grow  for  ages,  may  for  ages  cease 
to  grow — the  relief  of  the  pressure  which  led  to  its  construc- 
tion being  afforded  by  foldings  of  the  earth's  crust  at  other 
points,  sometimes  far  away  from  the  original  seat  of  disturb- 
ance. So,  too,  that  other  class  of  disturbingf  actions  involved 
in  the  formation  of  dykes  appears  to  be  only  locally  active 
in  any  geological  period,  though  in  the  succession  of  the 
ages  it  probably  affects  every  part  of  the  crust. 

In  the  present  condition  of  the  earth's  crust,  so  far  as  the 
brief  historic  record  goes  to  show,  earthquakes  of  an  Intensity 
menacing  to  man  are  limited  to  certain  regions,  which  prob- 
ably do  not  altogether  include  more  than  one-fourth  of  the 
area  of  the  lands,  though  shocks  of  a  less  degree  of  violence 
appear  to  be  common  to  every  part  of  the  surface  of  the 
continents.  The  regions  of  recurrent  shocks  of  considerable 
violence   are   so   irregularly   distributed   that   they  cannot   be 


l8  ASPECTS    OF  THE  EARTH. 

adequately  noted  in  this  brief  consideration  of  the  subject. 
They  include,  in  Europe,  Iceland,  Portugal,  Spain,  and  South- 
ern Italy  ;  the  region  of  the  Lower  Danube,  and  some  of  the 
islands  of  the  Grecian  Archipelago.  In  Asia,  the  larger  part 
of  Asia  Minor,  several  limited  areas  in  Hindostan,  the  greater 
part  of  the  eastern  littoral  region  of  Asia,  and  the  islands 
of  the  Japanese  and  Malayan  Archipelagoes  are  subjected  to 
destructive  shocks. 

In  Africa  there  is,  save  in  Egypt,  little  architecture  to 
suffer  from  earthquake  disturbance,  and  even  less  history  to 
record  it.  Egypt  seems  to  have  been,  on  the  whole,  sin- 
gularly exempt  from  great  earthquakes,  while  the  western 
portion  of  the  Mediterranean  face  of  the  continent  shares  the 
disturbances  from  which  the  Spanish  peninsula  has  repeatedly 
suffered.  The  vast  Australian  and  Polynesian  district  of  the 
Pacific  affords  a  number  of  regions  of  great  earthquake  activ- 
ity, of  which  New  Zealand  is  the  only  one  where  we  have 
anything  like  good  observations  for  even  a  few  score  years. 
It  may  be  said,  however,  that  the  greater  part  of  this  vast 
area  seems  to  be  more  exempt  from  these  indications  of 
activity  in  the  crust  than  any  other  equally  extensive  part 
of  the  earth's  surface. 

We  come  now  to  the  twin  continents,  North  and  South 
America.  The  obvious  resemblances  in  the  physical  config- 
uration of  these  continents  lead  us  to  expect  a  likeness  in 
their  conditions  of  stability.  This  resemblance  in  a  certain 
measure  exists.  The  w(;stern  shore  of  both  of  these  con- 
tinents, the  s(Naward  face  of  the  great  Cordilleran  range  of 
mountains,  is  tiie  scat  of  the  most  frefjuent  and,  on  the  whole, 
the  most  energetic  disturbances  which  occur  within  their 
limits,  while  the  eastern   shore   of  each   is  comparatively  little 


THE   STABILITY  OF  THE  EARTH.  1 9 

assailed  by  shocks.  The  northern,  or  Venezuelan,  district  of 
South  America,  which  is  apparently  the  seat  of  an  active 
mountain  growth,  of  which  there  is  no  parallel  in  the  northern 
continent,  is  a  district  of  recurrent  shocks  of  orreat  violence, 
such  as  have  never  been  observed  in  high  latitudes  on  our 
own  continent.  On  the  other  hand,  the  region  from  the 
mouth  of  the  Amazon  to  the  La  Plata  River,  which  corre- 
sponds to  our  sea-board  Atlantic  States  and  the  provinces  of 
Canada,  enjoys  an  immunity  from  disturbances  probably  not 
exceeded  by  any  other  equally  extensive  area  occupied  by  the 
Aryan  race,  while  the  corresponding  region  in  North  America 
is  much  less  fortunate. 

It  is  worth  our  while  to  look  more  closely  to  the  seismic 
history  of  North  America  than  we  have  been  able  to  do  in 
the  case  of  other  lands,  not  alone  because  of  our  momentary 
personal  interest,  but  because  it  is  in  the  future  to  be  the 
principal  dwelling-place  of  our  race  and  the  home  of  the  type 
of  civilization  which  that  race  is  developing. 

There  can  be  no  question  that  where  a  people  is  exposed 
to  recurrent  and  overwhelming  danger,  such  as  menaces  the 
inhabitants  of  Peru,  Venezuela,  or  Calabria,  a  danger  which  as 
yet  is  not  foretold  by  science  or  effectively  guarded  against 
by  art,  the  conditions  are  likely  to  tell  upon  its  character. 
"■  To  the  firm  eround  of  nature,  trusts  the  hand  that  builds  for 
aye,"  is  true  in  a  real  as  well  as  in  a  metaphoric  sense.  This 
trust  in  a  stable  earth  is  a  necessary  element  in  much  that  is 
noblest  and  most  aspiring  in  the  life  of  men.  Expose  ordi- 
nary people  to  constant  devastations  from  an  overwhelming 
force,  whether  it  be  in  the  form  of  a  human  enemy  or  a  natural 
agent,  and  their  state  of  mind  becomes  unfavorable  for  the 
maintenance  of    a  high  civilization.      The  best  conditions  of 


20  ASPECTS   OF  THE  EARTH. 

society  can  only  be  secured  when  the  laborer  toils  with  the 
assurance  that  his  work  will  endure  long  after  his  own  brief 
life  is  over. 

Care  must  be  taken  not  to  make  too  much  account  of 
the  effect  exercised  by  the  great  convulsions  of  nature  on  the 
moral  condition  of  a  people.  The  need  of  this  precaution 
is  well  shown  by  the  social  history  of  Iceland.  This  country 
has  for  the  thousand  years  of  its  history  been  subjected  to 
imminent  peril  from  the  instability  of  the  earth  as  well  as 
from  the  inhospitable  nature  of  its  climate.  In  almost  every 
century  of  the  world's  history  famine  caused  by  the  accidents 
of  the  earth  and  air  has  menaced  the  life  of  the  population. 
Many  successive  volcanic  outbreaks,  attended  by  serious  earth- 
quakes, have  convulsed  this  island,  and  yet  amid  these  mishaps 
the  people  have  maintained  the  highest  measure  of  social 
order  in  any  state  of  which  we  have  a  history.  The  Iceland- 
ers have  had  the  moral  strength  to  rise  superior  to  such 
afflictions.  In  this  state,  as  in  certain  individuals,  chastise- 
ment which  would  have  destroyed  weaker  natures  served  to 
affirm  the  vigor  of  the  strong  people. 

Earthquake  shocks  may  for  convenience  be  divided,  ac- 
cording to  the  violence  of  the  disturbance,  into  the  following 
classes  : 

Mrst.  the  shocks  of  extreme  intensity,  in  which  the  most 
perfectly  constructed  masonry  is  destroyed,  semi-detached 
masses  of  stones  along  the  faces  of  cliffs  thrown  down,  and 
the  soil-covering  of  the  earth  shaken  as  in  a  sieve.  Of  this 
group  the  greater  earthquakes  of  Peru  and  that  of  New 
Madrid,  Mo.,  may  serve  as  examples. 

Second,  shocks  of  great  intensity,  in  which  all  but  the 
strongest  edifices  are  overthrown,  frail  |)innacles  of  rock  over- 


THE  STABILITV  OF  THE  EARTH. 


21 


turned,  and  the  soil  frequently  rent  by  fissures.  In  this 
group  the  earthquake  at  Charleston  in  1886  may  find  a  place, 
though  it  probably  belongs  in  the  next  lower 
division. 

Third,  shocks  of  moderate  intensity,  when 
the  weaker  buildings  alone  are  seriously 
damaged,  and  the  natural  features  of  the 
surface  not  much  affected.  In  this  group 
we  may  fairly  place  the  Massachusetts 
earthquake  of  1755,  and  probably, 
as  just  suggested,  that  of  Charles- 
ton in  1886. 

Fourth,    shocks 
distinctly 
percept-   1 
ible     to    the 
senses,     but  - 
of     slight 
effect     even 
on    weak 
architecture, 
and  without    - 
distinct   in-     "^ 
fluence  upon 
the     natural 
features     of 
the      earth's 
surface. 

It  should  be  understood  that  these  divisions  are  merely  for 
convenience  of  description — in  fact,  earthquakes  form  a  continu- 
ous series,  grading  from  the  slightest  to  the  most  violent  shocks. 


^■^■^ 


,  y. 


-&^^ 

^     a 


Pinnacled    Rocks. 


Likely  to  be  overturned  by  a  succession  of  powerful  earthquakes. 
(U.  S.  Geological  Survey.) 


2  2  ASPECTS   OF  THE  EARTH. 

In  endeavoring  to  determine  the  degree  to  which  the 
different  parts  of  North  America  have  been  subjected  to 
devastating  earthquake  shocks,  or  to  those  which  would  prove 
disastrous  in  a  country  occupied  by  a  comphcated  society,  we 
find  ourselves  met  with  the  difficulty  which  arises  from  the 
brevity  of  our  historic  records,  concerning  the  greater  part  of 
this  continent.  It  is  true  that  in  Mexico,  and  the  peninsula 
district  to  the  southward,  we  have  a  record  which  comprises 
nearly  five  lumdred  years  ;  but  of  the  rest  of  the  continent 
our  longest  records  are  only  of  about  half  that  duration,  and 
these  concern  only  a  little  strip  of  country  along  the  Atlantic 
coast  of  the  continent  ;  for  the  remainder  the  information  is 
for  a  brief  term  of  a  single  century.  It  has  occurred  to  the 
present  writer  that  it  itiay  be  possible  to  supplement  this 
extremely  imperfect  historical  record  by  an  examination  of  the 
very  numerous  poised  blocks  as  well  as  the  detached  and 
frail  columns  of  stone  which  abound  in  many  districts,  natural 
monuments  which  would  be  overturned  by  a  succession  of 
great  earthquakes  as  easily  as  a  Gothic  steeple  or  other  frail 
work  of  human  architecture.  Although  little  has  been  done 
with  this  mt'thod  of  investigation,  it  will  be  possible  to  make 
some  use  of  it  in  extending  an  inquiry  which,  if  it  rested  on 
human  testimony  alone,  would  be  extremely  imperfect  and 
imsatisfactory. 

1  hese  natural  indices  of  a  quiet  earth  have  been  formed 
in  two  (litierenL  ways,  viz.  :  In  the  glaciated  districts,  which 
practically  comprise  the  northern  lialf  of  the  continent, 
including  all  of  New  England,  New  York,  a  great  part  of 
Prnnsylvania.  Ohio.  Indiana,  and  the  northern  tier  of  the 
Western  States  and  Territories,  to  the  Pacific,  as  well  as  all 
the  vast  territory  to   the   northward   of  the  United   States,  we 


THE   STABILITV  OF  THE  EARIH.  23 

often  find  perched  bowlders,  or  erratics,  left  upon  the  surface 
at  the  melting  of  the  glacial  sheet.      These  blocks  not  infre- 
quently were  dropped   in  positions  from  which  a  great  earth- 
quake shock  would  easily  dislodge  them ;  occasionally  we  find 
a  larcre  block  which,  when   the   ice  melted  away,  came   to  be 
lodged  on   supporting  stones,  or   on   the   summit   of  a   rocky 
hill,  in  a  very  insecure   position.      Yet   more  often    we    find   a 
spheroidal  block,  say  two  or  three   feet  in  diameter,  perched 
on  a  laro-er  bowlder.      In  great  part  these  poised  stones  have 
been  overturned  by  snow-slides  and  falling  trees;  those  which 
escape  these   mischances    have  often   fallen   a   prey  to  boys, 
who  take  a  natural  delight  in  assisting  gravitation  to  destroy 
such  monuments.     In   New  England  and  other  glaciated  dis- 
tricts, the  present  writer  has  observed  many  hundreds  of  such 
natural  indications  of  immunity  from  earthquakes.     The  other 
class  of  these  indicators  is   that  of  columns  or  other  unstable 
masses  of    rock  which  have  been    preserved,  while    the    sur- 
rounding rock  has  been  worn  away,  either  by   the  action  of 
rain  and  streams,  or,  more  rarely,  by  the  beating  of  the  ocean 
waves   when  the  sea  was   higher   than   it    is  at   present.      All 
these   pinnacled  rocks  date    from   times    which,  in    a  historic 
sense,  are  very  ancient,  perhaps  hundreds  of  times  as    remote 
as  the   first  written   records  of  this  continent.      The  most   of 
these  pillared  stones,  having  a  height  of  twenty  feet,  may  be 
safely  reckoned  as  of  an  age  of  at  least  twenty  thousand  years, 
and  thus  give  us  evidence  of  long-continued    immunity  from 
shocks  of  the   first  or  second  order  in  the   districts  in  which 
they  are  found. 

It  is  to  be  noted  that  many  of  the  pinnacled  rocks,  such 
as  are  figured  in  these  pages,  are  much  more  substantial 
than  they  seem,  and  that  they  may  on  that  account  survive 


24 


ASPECTS    OF  THE   EARTH. 


the  assault  of  a  single   shock  of   consider- 
able   violence,  just    as  detached    chimneys 
withstood  the  Charleston  shock  with   little 
injury.      But    it    seems    certain    that    these 
frail  and  time-worn  columns,  such  as  those 
figured  from   the   gorge  of    the 
W       Kentucky  River  or  from  Cum- 
^;AC<^;-|   berland    Gap,    could    not     have 
endured  the  frequent  and    vio- 
lent movements  to  which   they 
liteLi::^^    would   have    been    subjected    if 
^^f     they    occupied    a    region    liable 
to    great   earth- 
quakes. 

It  is  true  that 
in  those  regions 
where  these  pin- 
nacles stand  as 
wMt  n  e  ss  e  s  of  a 
quiet  earth,  the 
long  dormant 
movements  of  the 
nether  world  may 
at  any  time  be 
awakened.  But  it 
is  clear  that  where 
a  region  has  en- 
joyed an  immu- 
nity from  violent 
earthquake  shocks 
".^        '       ^  during  a  period  of 

Eroiion  Column.  ,  . 

From  the  cafton  of  the  Kentucky  River.  Ky.    (  Ky.  GeoloRical  Survey.)    tweuty        thOUSand. 


■*^>^'^c: 


THE  STABILITY  OF  THE  EARTH 


25 


years  or  more,  we  may  safely  trust  it  for  another  millennium. 
At    any    rate,    the    natural    evidence,    despite    its    occasional 


Pinnacle   Rock    at   Cumberland   Gap 
Likely  to  be  overturned  by  a  violent  earthquake. 

obscurity,  deserves  to  be   taken   into  account  along  with  the 
historic  record  of  the  earthquakes  of  any  country.. 

Yet  other  facts  concerning  the  force  with  which  seismic 
convulsions  have  operated  during  the  present  geological 
period   in  any  district  may  be  obtained   by  the   conditions   of 


26  ASPECTS   OF  7 HE  EARTH. 

the  soil  on  steep  slopes.  It  is  a  fact  well  indicated  by  many 
observations  that  a  vigorous  earthquake  serves  in  all  cases  to 
impel  the  detrital  materials,  soil  or  the  coarser  fragments  of 
decayed  rocks,  down  the  slopes  on  which  they  are  formed. 
Thus  in  the  great  earthquakes  of  the  last  century  in  Jamaica, 
the  steeper  slopes  of  the  mountains,  though  thickly  soil  cov- 
ered and  forest  clad,  were  hurled  down  their  declivities,  so 
that  in  a  few  minutes  the  mountains  were  stripped  at  once  of 
their  woods  and  the  materials  in  which  the  trees  grew,  leaving 
bare  rock  where  before  had  been  luxuriant  vegetation. 

Wherever  we  find  the  soil-coating  thickly  distributed  on 
slopes  having  a  declivity  of  more  than  30°,  we  may  be  sure 
that  the  region  has  been  exempt  from  great  shocks  during  the 
time  when  the  decomposed  materials  were  accumulating.  The 
soil-coating  is  in  all  cases  slowly  formed.  Frost  may  rapidly 
break  the  rock  up  into  angular  fragments,  but  to  reduce  the 
rubble  to  the  state  of  soil  requires  many  thousand  years. 
If,  with  this  general  thought  in  mind,  we  make  a  survey  of 
the  Appalachian  Mountains  or  the  steeper  hill-slopes  of  the 
Mississippi  valley,  we  become  satisfied  that  the  region  has 
been  exempt  from  earthquake  shocks  of  much  intensity  for 
many  thousand  years ;  combining  this  evidence  with  that 
before  referred  to,  derived  from  poised  bowlders  and  natural 
pinnacles,  we  are  able,  at  least  in  a  general  way,  to  determine 
the  earthquake  history  of  a  country  for  tens  of  thousands  of 
years  in  the  past. 

Proceeding  in  this  way,  by  combining  the  natural  and  the 
historic  evidence,  we  find  that  this  continent  is  as  diverse  as 
any  other  of  the  great  land  masses  in  the  distribution  of  the 
earthquakes  of  dangerous  intensity.  Leaving  out  the  dis- 
tricts of  Central  America  and  Mexico,  where  the  distribution 


THE   STABILITV  OF  THE  EARTH.  27 

of  shocks  is  extremely  complicated,  and  where  they  are  not 
likely  to  be  a  matter  of  practical  importance  to  our  English 
race,  we  may  advantageously  consider,  first,  the  Atlantic  sea- 
board region  ;  then,  in  succession,  the  Mississippi  valley  and 
the  Great  Lakes  basin,  the  Rocky  Mountain  district,  and,  lastly, 
the  border  region  of  the  Pacific, 

For  our  purpose  it  is  necessary  to  divide  the  Atlantic  sea- 
board reeion  of  North  America  into  a  number  of  districts  : 
First  of  these  is  the  country  north  of  the  St.  Lawrence,  a 
district  doomed  to  sterility,  where  earthquakes  might  well  be 
allowed  to  rage,  but  which  appears  to  be  exempt  from  such 
disturbances.  In  the  southern  part  of  this  region,  on  the 
Mingan  shore  of  Labrador,  there  are  many  slender  columns 
of  rock  which  attest  a  long-continued  exemption  from  earth- 
quake shocks. 

Next  we  have  the  maritime  provinces  of  Canada,  which, 
by  the  historical  as  well  as  the  natural  evidence,  appear  long 
to  have  enjoyed  an  equal  freedom  from  severe  shocks.  Still 
farther  south  we  have  the  New  England  district,  extending 
from  the  Bay  of  Fundy  to  the  Hudson.  This  region,  from 
the  natural  evidence,  appears  to  have  been  pretty  generally 
exempt  from  severe  shocks  ;  this  evidence  is  clearest  on  the 
coast  of  Maine,  where  there  are  numerous  poised  bowlders 
and,  on  Mount  Desert,  occasional  columnar  masses,  detached 
by  the  action  of  the  sea,  where  many  thousand  years  ago  it 
stood  at  higher  levels  than  it  does  at  present.  The  other 
parts  of  New  England  afford  frequent  poised  blocks,  which 
lead  us  to  the  conclusion  that  the  whole  of  this  district  has, 
since  the  glacial  time,  escaped  severe  earthquakes,  though 
*the  evidence  on  this  point  is  less  conclusive  than  in  the 
region  along  the  shore  to  the  northward. 


28 


ASPECTS   OF  THE  EARTH. 


It  is  to  be  noted,  however,  that  since  the  settlement  of 
this  Xew  England  country  there  have  been  several  shocks  of 
an  alarm  in i^^  nature,  which  have  principally  affected  the  State 


Steepled  Rocks  and  Erosion  Bowlder. 
Indicating  exemption  of  district  frt)m  violent  earthquakes.     (U.  S.  Geological  Survey.) 

of  Massachusetts.  That  of  1727  and  several  following  years 
was  ont:  of  the  most  peculiar  disturbances  which  have  ever 
been  recorded.  The  first  movements  of  this  long-continued 
series  of  shocks  disturb(,'d  a  tolerably  large  area  ;  but  in  a 
short  tinn;  the  shocks   became  confined  to  the  region  near  the 


THE  STABILITI'  OF  THE  EARTH.  29 

old  town  of  Newbury,  Mass.,  where  from  1727  to  1740  each 
shock,  though  the  motion  was  sHght,  was  accompanied  by  loud 
and  terrifying  sounds  proceeding  from  the  depths  of  the  earth. 
We  have  the  story  of  this  strange  convulsion  from  the  journal 
of  the  Rev.  Matthias  Plant,  the  pastor  of  the  Puritan  church 
at  Newbury.  Although  he  viewed  the  matter  rationally, 
many  people  believed  that  the  tumult  was  caused  by  the  devil 
at  work  in  the  nether  realm. 

In  1755,  almost  coincidently  with  the  great  Lisbon  earth- 
quake, central  New  England  was  visited  by  a  disturbance  of 
considerable  violence,  one  which,  though  a  single  shock,  was 
probably  nearly,  if  not  quite,  as  violent  as  any  of  the  several 
movements  which  occurred  in  South  Carolina  in  1886.  This 
disturbance,  though  not  hurtful  to  life  or  limb,  did  a  good 
deal  of  minor  damage  to  the  buildings  of  Boston  and  vicin- 
itv  ;  a  good  part  of  the  chimneys  were  overturned,  and 
wherever  a  heavy  weight  was  supported  on  a  tall,  frail  base 
the  effects  were  considerable.  John  Winthrop,  then  professor 
of  physics  and  astronomy  in  Harvard  College,  one  of  the  few 
eminent  American  men  of  science  of  the  eighteenth  century, 
states  that  the  bricks  from  the  chimney  of  his  house,  in  Cam- 
bridge, the  top  of  which  was  thirty-two  feet  from  the  ground, 
were  thrown  to  a  point  thirty  feet  from  the  base  of  the  struc- 
ture. If  we  may  trust  this  observation,  it  is  clear  that  the 
shock,  though  not  of  great  violence,  was  of  sufficient  force  to 
brine  havoc  to  many  flimsy  structures  of  the  present  day. 
Since  1755  there  has  been  no  earthquake  in  this  district 
which  can  be  termed  menacing  in  its  violence,  though  move- 
ments of  slight  importance  have  been  numerous. 

We  may  reasonably  conclude  that  while  the  New  England 
district  has  probably  long  been  exempt  from  disturbances  of 


30 


ASPI'CTS   OF  THE  EARTH. 


great  severity,  the  Massachusetts  district  appears  to  be  Hable 
to  shocks  of  a  violence  sufficient  to  wreck  buildings  which 
are  not  well  fitted  to  sustain  such  assaults. 

From   the    Hudson   southward   to   the   James    River,  and 
westward   to   the   meridian   of   Cincinnati,   we  have  a   region 


Erosion    Arch. 
Showinp  a  type  of  structure  likely  to  be  destroyed  by  powerful  shocks. 

which,  from  the  natural  as  well  as  the  historic  evidence,  we 
may  consider  as  far  free  from  earthquake  action  of  a  danger- 
ous sort  as  any  part  of  the  United  States.  In  this  region 
frail  pinnacled  rocks  and  those  remnants  of  old  caves,  the 
so-called  iiahii-al  i>?'i((ocs,  themselves  often  very  frail,  abound, 
and  afford  good  evidence  that  earthquakes  of  great  force, 
those  which  we  have  classed  as  of  the  first  and  second  order, 
have  for  many  thousand  years  been  wanting  in  this  district. 
Moreover,   the  historic  evidence  goes  to    show  that    for   two 


THE   STABILITl     OF   THE  EARTH.  3 1 

centuries  or  so  there  have  been  no  disturbances  of  importance 
in  this  region. 

South  of  the  James  River  we  enter  upon  the  wide  low- 
lands of  the  Atlantic  shore.  In  this  region,  owing  to  the  low 
nature  of  the  topography,  there  can  be  none  of  the  natural 
shock-indicators  which  we  have  sought  to  use  in  exploring 
the  past  history  of  earthquakes.  Parts  of  this  region  have 
been  twice  shaken  with  considerable  violence — first  in  the 
earthquake  of  1811,  which  mainly  affected  a  small  area  on 
the  borders  of  the  Mississippi,  but  propagated  its  waves  to 
this  part  of  the  Atlantic  sea-board  ;  and  again  in  the  Charles- 
ton earthquake  of  1886.  That  of  Charleston,  though  in  vio- 
lence not  to  be  compared  with  the  greater  shocks  of  South 
America,  Jamaica,  or  Central  America,  was,  next  to  that  of 
181 1,  the  most  violent  which  within  the  historic  period  has 
ever  affected  any  part  of  the  United  States  east  of  the  Rocky 
Mountains.  Still  it  probably  should  be  classed  as  of  the 
third  order  in  violence.  If  the  edifices  of  Charleston  had 
been  built  according  to  the  rules  which  should  guide  archi- 
tects who  intend  to  guard  against  such  calamities,  it  seems 
certain  that  the  disastrous  consequences  of  that  shock  would 
have  been  avoided. 

The  sea-board  section  of  the  Gulf  States,  like  that  of 
the  Carolinian  region,  affords  us  no  satisfactory  geological 
evidence  as  to  its  earthquake  history.  But  the  mountainous 
region  of  the  southern  Appalachians,  wliich  is  not  far  removed 
from  this  district,  abounds  in  spire-shaped  rocks  which  are 
delicately  poised  on  their  bases,  and  appear  to  show  that  great 
shocks  have  long  been  unknowm  in  those  uplands.  They 
especially  abound  in  the  valleys  in  which  flow  the  upper 
tributaries  of  the  Tennessee  River.     The  greater  part  of  the 


-2  ASPECTS    OF  THE  EARTH. 

Mississippi  valley,  as  far  as  the  natural  and  historic  evidence 
goes  to  show,  appears  to  be  the  seat  of  but  slight  disturb- 
ances ;  but  in  the  central  portion  of  that  area,  from  the 
junction  of  the  Ohio  and  Mississippi  rivers  southward,  in  a 
re^non  where  the  natural  indices  of  the  earth's  stability  are 
wanting,  we  have  the  seat  of  the  greatest  disturbance  that  has 
been  recorded  on  any  part  of  this  continent  north  of  the 
district  of  the  isthmus. 

The  shocks  which  affected  the  Mississippi  valley  in  1811- 
13  are,  by  their  violence  and  continuity,  to  be  ranked  among 
the  first  score  of  recorded  earthquakes.  Save  perhaps  that 
which,  in  18 19,  disturbed  the  delta  of  the  Indus,  in  west- 
ern Hindostan,  the  Mississippi  earthquake  of  181  i  directly 
produced  more  extensive  and  permanent  local  geographical 
changes  than  any  other  of  which  we  have  an  account  ;  so  vio- 
lent and  continuous  were  the  shakings  that  the  alluvial  land 
in  the  neighborhood  of  New  Madrid  was  lowered  below  its 
previous  level,  and  into  the  depressed  region  the  stream  of 
the  Mississippi  poured  in  such  violence  that  for  a  time  its 
lower  waters,  for  a  considerable  part  of  their  course,  turned 
backward  toward  their  source.  Although  the  colonizing  of 
the  district  had  just  begun,  the  area  of  country  already  cleared 
by  settlers  which  was  converted  into  morasses  by  the  shock 
was  so  great  that  the  government  was  compelled  to  furnish 
some  hundretls  of  thousand  acres  of  new  lands  on  higher 
ground  to  lliose  whose  dwelling-places  had  been  made  unin- 
hai)itablc.  It  seems  likely  that  an  area  of  not  less  than  five 
thousand  scjuart;  miles  was,  on  the  average,  though  irregu- 
larly, lowered  to  th<-  depth  of  ten  feet  below  its  original  level. 
The  energy  of  these  shocks  was  so  great  that  the  low,  strongly 
built  cal)ins  of    th(!    jjioneers  were   wrecked,   the  forest   trees 


THE  STABILITY  OF   THE  EARTH.  33 

were  beaten  against  each  other,  and  their  branches  interlocked 
as  they  swung  to  and  fro.  The  irregular  movements  of  the 
o-round  led  to  the  formation  of  numerous  <Treat  crevices,  from 
which  turbid  waters  were  thrown  up  to  a  considerable  height. 
To  protect  themselves  from  being  engulfed  in  these  fissures, 
the  people  felled  trees  so  that  they  lay  on  the  ground  at 
rio-ht  angles  to  the  oreneral  trend  of  the  fissures,  and  built 
places  of  refuge  on  the  broad  foundations  which  they  thus 
secured.  There  can  be  no  question  that  a  disturbance  of 
this  magnitude  would,  in  the  present  condition  of  the  region 
where  it  occurred,  cause  greater  destruction  than  did  that  at 
Charleston. 

These  two  series  of  disturbances,  that  of  1811  and  1886, 
have  a  close  general  relation  to  each  other.  So  alike  are  they, 
indeed,  as  to  suggest  that  the  great  series  of  repeated  shocks, 
gradually  diminishing  In  intensity,  may  be  the  type  of  disturb- 
ance characteristic  of  the  lowland  districts  of  the  southern 
part  of  this  continent.  The  New  Madrid  earthquake  of  iSii 
was,  however,  by  far  the  more  extended  phenomenon  ;  the 
shocks  were  more  frequent  and  of  much  greater  violence,  and 
the  period  during  which  they  recurred  was  far  longer  than 
in  the  Carolinian  disturbances. 

The  peculiar  local  subsidence  of  the  land  which  occurred 
during  the  earthquakes  of  181 1  in  the  alluvial  region  of  the 
Mississippi  valley,  as  well  as  similar  accidents  In  other  like 
districts  in  various  parts  of  the  world  during  earthquake 
shocks.  Is  probably  to  be  attributed  to  the  fact  that  In  delta 
regions  the  frequent  changes  in  the  path  of  the  stream  form, 
in  the  manner  described  In  the  chapter  on  Rivers,  numerous 
lakes  In  the  abandoned  portions  of  the  stream-bed.  These 
basins  gradually  become   filled  with   accumulations   of  vege- 


34 


ASPECTS   OF  THE  EARTH. 


table  matter,  and  in  time  are  floored  over  by  the  river  mud» 
so  that  all  surface  indications  of  the  effaced  lake  disappear. 
The  thick  layer  of  vegetable  matter  gradually  decays,  the 
carbon  unites  with  the  oxygen  or  becomes  a  gas  and  escapes 
to  the  atmosphere  through  the  porous  covering  of  the  earth. 


\  I  '' 


f^ 


liiiii/**^'^ 


W»  J 


„^v,-o. 


-^J 


4^1 


ij^?^'^^^'^ 


'■'^"-^j 


'v\ 


Spring    Hole. 

Formed  during  the  Charleston  earthquakes  of  i886.     Type  of  fissure  springs  formed  by  earthquakes  when 

the  soil  is  very  deep. 


W  hen  this  process  of  decay  has  gone  on  for  a  great  period, 
an  earthquake  shock  causes  the  mass  to  settle  together,  so 
that  the  surface  may  be  much  lowered.  If  this  view  be 
maintained,  we  may  then  find  in  the  great  subsidences  which 
occurred  in  the  Mississippi  valley  during  the  earthquake  of 
iSii  proof  that  that  shock  followed  on  a  very  long  period 
of  quiet  conditions  ;  for  if  there  had  been,  numerous  such  dis- 


THE   STABILITY  OF   THE  EARTH.  35 

turbances  in  the  past,  the  condensation  of  the  earth  would 
have  been  previously  accomplished.  Similar  evidence  is 
afforded  by  the  great  crevices  formed  in  the  Mississippi 
valley  in  the  earthquake  of  1811,  and  the  similar  fractures 
produced  in  the  Charleston  shock  of  1886,  such  as  are  fig- 
ured on  the  page  opposite.  Similar  fractures  of  small  size 
occur  in  all  countries  violently  shaken  by  earthquakes,  even 
where  such  shocks  are  frequently  repeated  ;  but  extensive  frac- 
tures of  this  sort  in  a  level  country  would  intimate  that  the 
reo-ion  In  which  they  occur  had  not  been  disturbed  by  previ- 
ous shocks  for  a  great  period,  perhaps  for  tens  of  thousands 
of  years. 

North  of  the  Ohio  and  Missouri  rivers  we  have  no  his- 
toric record  of  decided  seismic  disturbances.  In  Ohio,  Indi- 
ana, and  Illinois  the  natural  evidence  is  obscure,  there  being 
few  detached  columns  of  rock  which  could  serve  as  indices  of 
the  past  condition  of  the  country.  In  the  district  about  the 
upper  Great  Lakes  the  natural  evidence  coincides  with  the 
historic  record  in  showing  that  great  disturbances  have  not 
occurred  in  that  section.  Of  the  region  of  the  great  plains, 
including  Texas,  there  is  no  information  of  much  value, 
though  in  an  indecisive  way  the  topographic  evidence  is  in 
favor  of  the  conclusion  that  it  has  not  .been  seriously  shaken 
for  a  considerable  period. 

The  topography  of  the  central  and  eastern  section  of  the 
Rocky  Mountains  gives  fairly  clear  evidence  that  the  surface 
of  that  region  has  been,  as  a  whole,  tolerably  exempt  from 
great  shocks.  The  light  rainfall  of  that  part  of  the  continent 
causes  the  erosion  which  produces  pinnacled  rocks  and  steep- 
walled  canons  to  take  place  in  a  much  less  rapid  manner  than 
in  the  Appalachians;  yet  parts  of  this  region  abound  in  such 


36  ASPECTS   OF  THE  EARTH. 

pinnacles,   which    are    evidently    very    ancient,    though    often 
extremely  susceptible  to  strong  shocks. 

The  western  coast-line  region  of  the  Cordilleras  district, 
from  northern  California  southward  to  the  Mexican  line,  is 
more  or  less  subjected  to  earthquakes  of  considerable  energy, 
as  is  shown  by  historic  records.  One,  in  1812,  destroyed  a 
church  in  Los  Angeles,  Cal,  killing  a  score  or  more  people. 
Together  with  the  Charleston  earthquake  this  shock  is  entitled 
to  a  peculiar  place  in  our  history  ;  these  two  shocks  being 
the  only  earthquakes  which  have  caused  any  loss  of  life  in 
this  country.  There  have  been  several  considerable  shocks 
in  the  region  about  San  Francisco,  of  which  that  in  October, 
186S,  caused  the  overthrow  of  many  frail  buildings,  and  led  to 
precautions  in  the  construction  of  important  edifices  which 
seem  likely  to  insure  them  from  serious  accidents. 

The  vast  district  of  the  North  American  Cordilleras  con- 
tains so  many  separate  centres  of  action  of  the  mountain- 
building  and  volcanic  forces,  which  have  evidently  been,  in 
some  cases,  active  in  very  recent  times,  that  it  will  not  do  to 
extend  the  conclusions  obtained  from  poised  and  pinnacled 
rocks  very  far  from  the  places  where  these  features  occur.  It 
may  well  be  the  case  that  many  limited  areas  in  this  field  are 
at  present  liable  to  shocks  of  a  severe  nature. 

This  brief  and  unsatisfactory  review  of  the  seismology  of 
North  America  clearly  indicates  that  while  the  region  of  the 
United  States  and,  we  may  say,  of  the  habitable  part  of  the 
continent  north  of  Mexico  has  many  districts  which  are  sub- 
jected to  earthquake  shocks  of  moderate  intensity,  by  far  the 
greater  part  of  its  surface  shows,  within  the  narrow  limits  of 
historic  records,  no  evidence  of  great  seismic  dangers,  and 
indicates  by  its  topographical  features  that   it  has  long  been 


Crack    in   the   Ground. 
Produced  by  the  Charleston  earthquake  of  1886. 


THE  STABILITY  OF   THE  EARTH. 


Z7 


preserved  from  the  action  of  very  violent  shocks.  The  only 
region  which  we  can  say  has  ever  been  exposed  to  shocks  of 
anything  like  the  first  magnitude  is  a  district  probably  includ- 
ino-  an  area  of  not  exceeding  twenty  thousand  square  miles, 


Navaho   Church"    Pinnacled    Rocks. 
Likely  to  be  overturned  by  a  succession  of  earthquakes  of  the  second  order  of  violence. 

with  its  centre  about  fifty  miles  below  the  junction  of  the 
Ohio  and  the  Mississippi  rivers.  Shocks  of  the  second  order 
are  almost  equally  rare  ;  those  of  California  in  1S12  and  1868 
may  have  been  of  this  degree  of  force,  but  the  evidence  is  too 
incomplete  for  accurate  determination.  Those  of  the  lesser 
order,  but  still   of  a   degree   calculated   to   be   destructive   to 


93512 


-S  ASPECTS   OF  THE  EARTH. 

weak  architecture,  are  more  common;  that  of  1755  in  New 
England,  several  on  the  Pacific  coast,  and  that  of  Charleston, 
S.  C,  may  be  placed  in  this  category. 

Limiting  ourselves  to  historic  evidence  alone,  we  may  con- 
sider that  shocks  of  the  third  degree  of  violence  are  likely  to 
happen  in  central  New  England,  the  Pacific  coast  south  of 
Oregon,  and  in  the  southern  lowlands  of  the  United  States, 
and  are  probably  to  be  expected  in  other  areas.  The  natural 
evidence,  though  it  clearly  indicates  that  the  more  violent 
shocks  have  not  been  common  in  the  larger  part  of  our 
territory,  does  not  show  that  these  minor,  but  still  possibly 
devastating,  shocks  were  wanting. 

Although  there  are  no  natural  monuments  in  the  low- 
land region  of  the  Mississippi  which  serve  us  as  proof  as  to 
the  violence  of  the  seismic  power  in  prehistoric  times,  there 
seems  to  be  some  evidence  to  show  that  the  great  disturbance 
of  iSii  was  exceptional  in  its  nature,  and  not  a  frequently 
recurrent  phenomenon  in  the  region  where  it  took  place.  The 
remarkable  settlement  of  the  soil,  which  was  the  most  con- 
spicuous feature  among  the  effects  of  this  shock,  was  probably 
due  to  the  fact  tliat  the  alluvial  deposits  covering  the  country 
in  which  it  occurred  were,  from  tlie  circumstances  of  their  for- 
mation, very  open  structured,  and  became  condensed  by  the 
shaking  to  which  they  were  subjected,  just  as  any  other  loose 
earth  compacts  when  frequently  jarred.  If  this  were  the  case, 
and  there  are  many  facts  to  prove  it  which  cannot  be  dis- 
cussed here,  then  we  may  presume  that  ages  of  comj)arative 
quiet  had  gone  by  during  which  this  unconsolidated  allu\ial 
matter  was  fonuing,  and  that  ages  may  again  elapse  before  a 
similar  accident  recurs  in  that  region. 

We  should  note  the  fact  that  over  the  surface  of  the  world 


THE   STABILITV  OF  THE  EARTH.  39 

in  general  the  great  earthquakes  do  not  often  sporadically 
occur,  thouofh  there  are  some  cases  of  considerable  disturb- 
ances  which  have  not  been  repeated,  even  after  many  centu- 
ries. Thus  the  shock  which  in  the  year  1185  overthrew  the 
cathedral  at  Lincoln  in  England,  that  which  in  1208  in  good 
part  destroyed  the  cathedral  at  Wells,  and  that  which  in  1510 
destroyed  the  town  of  Nordlingen  in  Bavaria,  are  the  only  his- 
toric shocks  of  great  force  which  have  affected  the  regions  in 
which  these  accidents  occurred.  It  may,  perhaps,  reasonably 
be  hoped,  though  it  cannot  fairly  be  reckoned,  that  the  shocks 
of  New  Madrid  and  Charleston  were  in  the  nature  of  such 
isolated  disturbances. 

It  is  satisfactory  to  find  that,  within  the  area  of  the  United 
States,  two  centuries  of  historic  record  and  much  natural 
evidence  go  to  show  that  great  earthquakes  are  exceptional  ; 
but  this  should  not  blind  us  to  the  fact  that  large  areas  are 
already  known  to  have  suffered  from  movements  which  may 
bring  wide-spread  destruction,  where  the  builder  takes  no 
account  of  any  other  disturber  of  stability  save  gravitation. 
It  is  not  likely  that  we  as  yet  know,  by  experience,  the  full 
extent  of  country  which  is  subject  to  this  order  of  shocks  : 
our  historic  perspective  is  very  short,  and  the  natural  evidence 
does  not  give  us  any  assurance  concerning  disturbances  of 
this  lesser  order.  It  is  clear  that  we  cannot,  in  this  country, 
reckon  on  an  earth  as  stable  as  that  of  the  northern  region 
of  Europe,  where  our  race  was  bred  and  our  building  system 
developed.  It  is  equally  clear  that  the  mode  of  construction 
should  be  adapted  to  the  new  needs  which  the  less  firm 
ground  of  this  country  imposes  on  us. 

As  lone  as  the  building  material  most  commonly  in  use 
was   timber,    and   the   masonry  structures  of  a  low  and  sub- 


40 


ASPECTS   OF  THE  EARTH. 


stantial  nature,  they  were  fairly  fitted  to  afford  the  resistance 
required  to  withstand  the  shocks  which  could  be  expected  to 
come  upon  them.  But  the  combination  of  ambition  and 
economy  which  is  filling  the  land  with  lofty  and  flimsy 
structures    invites   calamity   on    the   least   disturbance   of    the 


Street   m    Charleston. 
Showini,'  ihe  relative  effect  of  a  m(jderately  strong  earthquake  on  timber  and  masonry  buildings. 

earth.  The  shock  of  1755,  which  did  little  more  than  stir 
the  fears,  shake  down  the  chimney-tops  of  the  old  town  of 
Boston,  and  afford  a  text  for  many  interesting  sermons,  would 
be  extremely  disastrous  to  the  higher  and  weaker  structures 
of  to-tla)'. 

The   prescriptions   which    the   architect    has    to    follow    in 
preparing   his   buildings    to   resist   the    strains  of  a  moderate 


THE  STABILITY  OF  THE  EARTH.  4 1 

earthquake  are  simple,  and  do  not  require  any  great  increase 
in  the  cost  of  construction.  It  is  well  to  understand  that  the 
actual  movement  of  the  ground,  even  in  violent  shocks,  is 
slieht.  In  those  which  we  have  termed  of  the  first  order 
it  is  doubtful  if  the  movement  ever  amounts  to  a  foot  in 
amplitude,  while  the  shocks  which  we  may  anticipate  in  this 
country,  such  as  have  recently  occurred  in  Charleston  for 
instance,  probably  swing  the  earth  to  and  fro  within  the  space 
of  an  inch.  The  destruction  is  done  in  part  by  the  sudden- 
ness of  the  to-and-fro  motion,  which  breaks  the  foundation 
from  the  superstructure,  but  in  larger  measure  by  the  pen- 
dulum-like vibration  which  is  set  up  in  the  building.  This 
pendulum  movement  may  cause  an  oscillation  of  one  inch  at 
the  foundations  to  be  several  feet  in  a  sixth  floor,  or  say  one 
hundred  feet  above  the  ground.  The  rending  effect  of  this 
pendulum-like  swinging,  especially  in  weak  masonry,  may 
easily  be  imagined. 

Many  well-considered  directions  for  the  protection  of 
buildings  from  earthquake  shocks  have  been  given  :  of  these 
the  best  may  be  found  in  the  excellent,  though  imperfectly 
phrased,  work  on  earthquakes  by  Professor  John  Milne,  of 
Tokio  University,  Japan.*  From  these  directions  we  extract 
the  following,  which  seem  applicable  to  our  conditions  : 

1.  "So  arrange  the  openings  in  a  wall  that  for  horizontal  stresses  the  wall 
shall  be  of  equal  strength  for  all  sections  at  right  angles." 

2.  "  Place  lintels  over  flat  arches  of  brick  or  stone." 

3.  "  Let  all  portions  of  a  building  have  their  natural  periods  of  vibration 
nearly  equal." 

4.  "Avoid  heavy-topped  roofs  and  chimneys." 


*  "  Earthquakes  and  Other  Earth  Movements."     By  John  Mihie.      ([nternational 
Science  Series.)     New  York  :  D.  Appleton  &  Co.     1886. 


42  ASPECTS   OF  THE  EARTH. 

5.  ••  In  brick  or  stone  vork  use  good  cement." 

6.  "  Let  archways  curve  into  their  abutments." 

7.  "  Let  roofs  have  a  low  pitch,  and  their  tiles,  especially  upon  the  ridges, 
be  well  secured." 

It  is  also  Important,  where  the  prevailing  direction  of 
motion  of  the  shocks  is  known,  to  have  the  blank  walls  of  the 
house  placed  so  as  to  be  parallel  to  the  course  of  the  shocks. 
It  is  also  worthy  of  note  that  generally  hill-tops  are  more 
shaken  than  the  ground  at  the  base,  for  the  same  general 
reason  that  the  upper  part  of  a  house  swings  more  during  a 
shock  than  the  basement.  Last  of  all,  the  higher  the  edifice 
the  more  risk  of  disastrous  oscillation  and  the  more  need  of 
binding  its  parts  firmly  together. 

Besides  the  immediate  effect  of  earthquakes  on  the  surface 
of  the  land  there  are  certain  secondary  consequences,  of  impor- 
tance to  man,  arising  from  the  action  of  the  sea  when  consid- 
erable shocks  originate  beneath  its  floor.  When  a  strong 
disturbance  is  produced  beneath  the  sea-floor  it  is  propagated 
for  a  ereat  distance  throuorh  the  water  in  exactly  the  same 
way  as  it  is  through  rock.  When  a  ship  is  near  above  the 
point  where  the  earthquake  occurs  her  people  feel  a  sensa- 
tion as  if  the  vessel  had  run  upon  a  rock.  The  vessel  may  be 
dismasted  or  her  seams  opened  by  the  blow.  There  are  many 
stories  extant  which  recount  the  narrow  escape  of  vessels 
from  destruction  by  these  submarine  earthquakes,  and  it  seems 
most  probable  that  many  good  ships  which  have  disappeared 
in  the  deep  have  been  overwhelmed  by  such  calamities. 

The  most  important  results  of  great  earthquakes  beneath 
the  sea  are  tlie  broad  waves  which  they  produce;  waves  which 
may  run  for  thousands  of  miles  before  they  break  upon  the 
shore.     We  may  fairly  represent  the  formation  of  these  waves 


THE   STABILITF  OF    THE  EARTH. 


43 


by  a  simple  expe^riment.  Taking  a  rtat-bottomed,  wide  pan,  of 
any  sheet  metal,  partly  filled  with  water,  let  us  strike  a  sharp, 
upward  blow  upon  its  base.  We  see  that  the  water  rises  in 
the  centre  and  rolls  off  in  a  broad  circular  wave  toward  the 
margin.      In  the  seas  this  wave  may  have  a  diameter  of  some 


\§:}  ^m^'^^^J^-^^^ 


_  J.  a.l  I ■ ^^-^•^-^■^■^-^ife-  .^^.-rm^-^:^^-'^^^^: 

Effect   of  a   Powerful    Earthquake  on   Massive    Masonry,   Italy. 

scores  of  miles,  though  its  height  probably  never  exceeds  a 
few  feet.  It  is  so  wide  and  low  that  as  long  as  it  is  in  deep 
water  it  may  slip  unnoticed  beneath  a  ship  ;  but  when  the 
front  edge  of  the  wave  comes  into  the  shallows  near  the 
shore,  its  advance  is  somewhat  retarded  by  the  friction  of 
the  bottom,  while  the  part  which  is  farther  out  to  sea  retams 


44  A  SPEC? S    OF  THE  EARTH. 

its  swift  motion.  The  wave  is  thus  crowded  into  a  less  space, 
and  so  becomes  constantly  higher  until,  when  it  rushes  on  the 
shore,  it  may  have  attained  a  height  of  fifty  feet  or  more.  These 
waves  are,  as  may  be  imagined,  exceedingly  destructive  :  on  the 
western  coast  of  South  America,  and  elsewhere,  they  constitute 
one  of  the  most  fearful  incidents  of  great  earthquake  shocks. 

It  is   a  matter  for  congratulation   that  the  coasts  of    the 
United  States  appear  to  be  exempt  from  disasters  of   this  na- 
ture.    Slight  movements  of  the  sea,  produced  in  the  manner 
above  described,  occasionally  visit  the  Pacific  shores  ;  but  they 
appear    to    be  derived   from  shocks  which  have  taken    place 
at  great  distances  from  that  coast-line.     The  Atlantic  shore 
of  the  United  States,  and  indeed  the  whole  shore-line  of  that 
ocean  north  of  the  Antilles  and  of  Portugal,  appear  to  be  free 
from  this  danger.     The  present  writer  has  observed  along  the 
rocky  portion  of  the  Atlantic  shore,  from   New  York  to  Nova 
Scotia,  a  great  number  of  delicately  poised  blocks,  resting  at 
a  heicrht  a  little  above  the   present  level  of  the  surf,  which 
clearly  indicate  that,  for  a  very  long  period   in  the  past,  this 
coast  has  been   free  from  such  violent  incursions  of  the  sea. 
Similar    and    even    more    conclusive    evidence,    to    show    the 
exemption  of  this  shore  from  these  violent  invasions  of  the 
sea,  is  afforded  by  the  delicately  moulded  surfaces  of  glacial 
debris  which  are  found  just  above  high  water  along  the  Atlan- 
tic coast,  from   New  Jersey  northward.     These  curiously  com- 
bined ridges  and  pits,  termed  Ijy  geologists  kavics,  are  almost 
as   frail  as  footprints  on  the  sand.      They  could  not  have  sur- 
vived a  single   tlooding  by  sucii   resistless  waves.      Thus   the 
natural  as  well  as  th<:  liistoric  evidence   points  to   the  conclu- 
sion   that   the    North   Atlantic   sea-bed   is   not  at    present  the 
seat  of  violent  earth(|uakes. 


THE  STAB  I  LIT y  OF  THE  EARTH.  45 

From  my  own  observations,  I  am  inclined  to  believe  that 
the  European  coast  of  the  Atlantic  Ocean  affords  sufficient 
evidence  to  justify  the  assertion  that  marine  waves  produced 
by  earthquakes  have  not  swept  upon  the  coast  in  the  region 
north  of  Spain.  Although  the  evidence  is  less  clear,  it  is  of 
the  same  nature  as  that  obtained  along  the  eastern  coast  of 
North  America,  and  similarly  entitles  us  to  consider  this 
region  as  exempt  from  great  convulsions  of  this  nature. 

We  may  sum  up  the  foregoing  considerations  as  follows  : 
The  continent  of  North  America  north  of  Mexico  seems, 
from  historic  as  well  as  natural  evidence,  to  be  in  the  main 
free  from  any  considerable  danger  of  earthquakes  which  are 
necessarily  destructive  to  architecture.  Nevertheless,  a  large 
part  of  its  surface  appears  to  be  liable  to  shocks,  which  though 
slight  may  be  very  destructive  to  life  and  property,  if  we  per- 
sist in  our  present  flimsy  methods  of  architectural  construc- 
tion. Good  fortune  has  given  us  a  tolerably  safe  abiding- 
place  for  our  race  in  this  country.  We  can  almost  everywhere 
safely  put  our  trust  in  it,  provided  we  are  willing  to  take  some 
care  as  to  methods  of  constructing  buildings. 

When  we  consider  the  magnitude  of  the  work  done  by  the 
subterranean  forces,  we  are  impressed  with  the  slight  nature 
of  the  disturbance  by  which  their  activity  is  manifested  to  us. 
It  is  only  in  a  limited  portion  of  the  earth's  surface  where 
these  disturbances  are  a  serious  menace  to  man.  The  damage 
they  cause  to  human  life  is  far  less  than  that  brought  about 
by  war  or  preventable  disease  ;  and  the  injury  to  edifices, 
though  appalling  by  its  suddenness,  is  on  the  whole  less  detri- 
mental than  that  arising  from  bad  methods  of  construction. 


VOLCANOES. 


Uniformity  of  Action  of  Earth's  Machinery.— History  of  Vesuvius  ;  Period  of  Greek  Set- 
tlements ;  Earthquakes  of  a.  d  63  ;  Eruption  of  A.  D.  79  ;  Story  of  the  Death  of 
Pliny  ;  Changes  Produced  by  this  Eruption  ;  Herculaneum  and  Pompeii  ;  Eruptions 
after  79. — Present  Condition  of  Vesuvius  ;  Observations  on  Eruption  of  1S82  ;  Lessons 
concerning  Volcanic  Eruptions  from  this  Eruption.  Other  Italian  Volcanoes.  —Icelandic 
Volcanoes  ;  Eruption  of  Skaptar  in  17S3  ;  Effect  on  Aspect  of  Sky.— Volcanoes  about 
Pacific  Ocean  :  Malayan  Volcanoes. — Eruption  of  Krakatoa  in  1S83.— Cause  of  Volcanic 
Eruptions  :  Method  of  Inquiry. — Distribution  in  Space  and  Time.— Daubree's  Experi- 
ment.— Effect  of  Accumulation  of  Strata.— Comparison  with  Blast  Furnace  ;  with 
Natural  Gas  Wells. — Evidence  from  ^Etna. — Comparison  of  Lunar  and  Terrestrial  Vol- 
canoes.— Effects  of  Volcanic  Action. 

The  greater  part  of  the  earth's  machinery  operates  in  a 
(ILiiet  manner,  with  something  hke  the  order  of  movement 
which  we  associate  with  the  motions  of  the  celestial  bodies. 
Steadfastly,  and  without  violence  of  a  perturbing  kind,  the 
continents  and  mountain-chains  rise  up,  the  rivers  and  seas 
wear  them  down,  and  from  age  to  age  the  great  procession  of 
life  moves  onward.  That  man  is  here  to-day  as  the  summit 
and  crown  of  all  the  life  through  which  he  has  come  to  his 
present  state,  is  suf^cient  evidence  that  the  terrestrial  powers 
have  never  worked  with  such  violence  as  to  throw  the  delicate 
mechanism  of  organic  life  out  of  adjustment.  If  we  could 
conceive  the  gigantic  nature  of  the  forces  which  act  upon 
and  witliin  the  earth,  this  orckn-  and  harmony  of  the  earth's 
machinery  would  appear  to  be  one  of  its  most  startling  feat- 
ures. It  is  only  in  volcanoes  that  we  may  see  something 
of   the  titanic  energies  of  the  universe.     They  alone  show  us 


VOLCANOES. 


M 


63    A.  D. 


by  what  delicate  adjustments  of  strengths  and  strains  this 
frail  mantle  of  life  is  enabled  to  maintain  itself  on  the  sur- 
face of  the  sphere. 

Although  the  popular  accounts  of  volcanic  eruptions  give 
the  general  reader  some  idea  of  the  great  energy  of  these 
catastrophes,  they  afford  no  adequate  conception  of  the  nature 
of  the  operations  which  constitute  these  outbreaks.  Still  less 
do  they  afford  him  any  knowledge  of  the  history  of  the  craters 
from  which  these  discharges  take  place.  We  will,  therefore, 
begin  our  inquiry  with  a 
brief  outline  of  what  is 
known  concerning  the  his- 
tory of  Vesuvius,  the  one 
volcano  of  which  we  have  a 
tolerably  full  account  for  a 
period  of  over  two  thousand 
years. 

The  reader  will  remem- 
ber that  Vesuvius  is  situated 
on  the  shores  of  the  Bay  of 
Naples.  This  part  of  the 
Italian    coast    affords    excel- 

1,11  „U^^.^."»,  ^     Diagrammatic  Sections  through   Mount  Vesuviu 

lent         harbors,        a       Charmmg       „J  changes  in  the  form  of  the  Cone.    (From  P 

climate,    and    a    fertile    soil. 

Moreover,  it  has  within  its  broad  expanse  a  number  of  islands 
which  in  the  early  days  afforded  admirable  strongholds  for 
the  small  colonies  of  the  Greek  folk  who  for  centuries,  in 
a  milder  way,  played  the  part  of  the  Scandinavians  of  the 
later  time  in  the  northern  seas.  The  island  of  Ischia,  lying 
upon  the  western  border  of  the  bay  which  was  in  time  to 
receive  its  name  from  the  relatively  modern  city  of  Naples, 


s,   show- 
(From  Phillips.) 


48  ASPECTS   OF  THE  EARTH. 

was  in  the  fifth  century  B.C.  the  first  seat  of  this  Grecian  set- 
tlement. At  that  time,  and  for  about  six  centuries  afterward, 
the  volcanic  cone  of  \'esuvius  was  not  in  activity  and  had  a 
very  different  aspect  from  that  it  has  in  the  present  day.  It 
was,  as  is  shown  in  the  cut  on  the  preceding  page,  a  broad, 
low  mountain,  not  rising  more  than  two  thousand  feet  above 
the  level  of  the  sea.  The  crater  was  deep  and  wide,  and  to  a 
modern  eye  would  have  told  its  volcanic  history  b)'  its  form  ; 
but  this  history  had  not  been  unravelled,  and  to  the  people 
of  that  time  it  was  a  hill  and  nothing  more. 

Durino-  the  lone  sleep  of  Vesuvius  the  settlers  on  Ischia 
were  afflicted  with  very  serious  eruptions  from  the  craters  on 
that  island,  and  at  one  time  were  driven  away  from  their  set- 
tlements by  these  disasters.  In  this  period,  while  Vesuvius 
was  at  rest,  there  were  perhaps  other  slight  eruptions  of  vol- 
canic gases  in  the  country  west  of  Vesuvius  known  as  the 
Phlcegrean  Fields.  It  is  now  evident  that  the  pent-up  vol- 
canic powers  were  struggling  to  open  another  way  for  their 
exit.  They  were,  however,  so  unsuccessful  that  the  country 
remained  for  centuries  but  little  disturbed.  It  became  the 
country-seat  of  the  wealthy  Roman  citizens,  who  found  there 
exemption  from  the  distractions  of  the  capital.  Around 
Vesuvius  itself,  along  the  shore  of  the  bay,  and  on  the  vine- 
clad  slopes  of  the  mountain,  there  were  wealthy  towns,  tem- 
ples, baths,  and  all  the  other  rich  constructions  of  that  archi- 
tecture-loving people,  the  Romans.  Except  for  the  eruptions 
in  Ischia.  which  was  sufficiently  remote  from  the  mainland  to 
make  its  disturbances  of  no  great  importance,  this  Vesuvian 
(hstrict  enjoyed  an  undisturbed  tranquillity  down  to  the  year 
63  of  our  era.  In  that  year  there  began  a  series  of  moderately 
stron*'^  earthcjuakcs   produced   by  the  volcanic  gases   in   their 


VOLCANOES. 


49 


struggle  to  reopen  their  long-closed  passages  to  the  crater.  In 
August,  79,  these  subterranean  movements  became  more  and 
more  violent  until  they  terminated  in  a  furious  eruption. 

We  gain   all  our   knowledge  of  the  circumstances  of  this 
great  catastrophe  from  the  letters  of  the  younger  Pliny  to  the 


\Q''L 


•  '''^1^^ 


Diagrammatic  Section  through  Vesuvius,  in  Time  of  Eruption,  showing  the  General  Form  of  the  Vapor- 
column  and  the  Falling  Ashes  and  Rain. 
The  lower  cloud  of  steam  is  from  lava-flows     The  lower  cup  of  the  crater  is  that  formed  before  the 
Christian  era. 

historian  Tacitus,  in  which  that  writer  gives  an  account  of  the 
death  of  his  uncle,  the  naturalist  Pliny,  who  lost  his  life  during 
the  eruption.  The  elder  Pliny  was  admiral  of  the  Roman  fleet 
stationed  in  the  port  of  Misenum,  now  known  as  Baiae,  on  the 
western  shore  of  the  bay.  The  eruption  began  about  mid- 
day, and  in  a  short  time  the  whole  of  the  eastern  side  of  the 


50  ASPEC7S   OF  THE  EARTH. 

bay  was  hidden  by  the  vast  cloud  of  steam,  commingled  with 
finely  pulverized  dust,  which  constitutes  the  so-called  smoke  of 
a  volcanic  eruption.  Gradually  this  cloud  extended,  until  it 
broucrht  the  darkness  of  nicrht  over  all  the  area  within  twenty 
miles  of  the  volcano,  and  a  wide  field  beyond,  extending  its 
shadow,  according  to  Dion  Cassius,  over  Africa,  Syria,  and 
Egypt. 

The  letters  of  the  younger  Pliny  were  designed  not  to 
give  a  detailed  account  of  the  eruption  itself,  in  which  the 
writer  seems  to  have  had  none  of  the  inquirer's  interest 
which  led  his  uncle  to  his  death,  but  to  give  Tacitus  informa- 
tion as  to  the  last  hours  of  the  great  naturalist.  His  account 
affords,  however,  though  incidentally,  a  picturesque  description 
of  the  catastrophe,  as  seen  by  a  cultivated  Roman  youth  of 
eighteen  years.  Notwithstanding  the  beauty  of  their  style 
and  their  charming  simplicity,  the  letters  of  the  younger  Pliny 
are  but  little  known  to  the  public,  even  in  translation.  I 
therefore  give  the  greater  part  of  the  two  which  refer  to  the 
eruption,  omitting  those  portions  which  contain  the  compli- 
ments in  which  Roman  correspondents  were  wont  to  indulge. 
This  translation  I  owe  to  my  friend.  Professor  J.  G.  Croswell, 
who  has  given  a  better  and  more  lively  rendering  of  the  text 
than  can  be  found  in  any  of  the  previous  versions. 

[Pliny's  Letters.     Book  vi.,  i6.] 

Gaius  Plinius  sends  to  his  friend  Tacitus  greeting. 

You  ask  me  to  write  you  an  account  of  my  uncle's  death,  that  ])osterity 
may  possess  an  accurate  version  of  the  event  in  your  history.     .     .     . 

He  was  at  Misenuni,  and  was  in  command  of  the  fleet  there.  It  was  at 
one  o'clock  in  the  afternoon  of  the  24th  of  August  that  my  mother  called  his 
attention  to  a  cloud  of  unusual  appearance  and  size.  He  had  been  enjoying 
the  sun,  and  after  a  bath  had  just  taken  his  lunch  and  was  lying  down  to 


VOLCANOES.  5  r 

read  ;  but  he  immediately  called  for  his  sandals  and  went  out  to  an  eminem  e 
from  which  this  phenomenon  could  be  observed.  A  cloud  was  rising  from 
one  of  the  hills  (it  was  not  then  clear  which  one,  as  the  observers  were  look- 
ing from  a  distance,  but  it  proved  to  be  Vesuvius),  which  took  the  likeness 
of  a  stone-pine  very  nearly.  It  imitated  the  lofty  trunk  and  the  spreading 
branches,  for,  as  I  suppose,  the  smoke  had  been  swept  rapidly  upward  by  a 
recent  breeze  and  was  then  left  hanging  unsupported,  or  else  it  spread  out 
laterally  by  its  own  weight,  and  grew  thinner.  It  changed  color,  sometimes 
looking  white,  and  sometimes,  when  it  carried  up  earth  or  ashes,  dirty  and 
streaked.  The  thing  seemed  of  importance,  and  worthy  of  nearer  investiga- 
tion, to  the  philosopher.  He  ordered  a  light  boat  to  be  got  ready,  and  asked 
me  to  accompany  him  if  I  wished  ;  but  I  answered  that  I  would  rather  work 
over  my  books.     In  fact  he  had  himself  given  me  something  to  write. 

He  Avas  going  out  himself,  however,  when  he  received  a  note  from  Rec- 
tina,  wife  of  Caesius  Bassus,  living  in  a  villa  on  the  other  side  of  the  bay,  who 
was  in  deadly  terror  about  the  approaching  danger  and  begged  him  to  rescue 
her,  as  she  had  no  means  of  flight  but  by  ships.  This  converted  his  plan  of 
observation  into  a  more  serious  purpose.  He  got  his  men-of-war  under  way, 
and  embarked  to  help  Rectina,  as  well  as  other  endangered  persons,  who 
were  many,  for  the  shore  was  a  favorite  resort  on  account  of  its  beauty.  He 
steered  directly  for  the  dangerous  spot  whence  others  were  flying,  watching 
it  so  fearlessly  as  to  be  able  to  dictate  a  description  and  take  notes  of  all  the 
movements  and  appearances  of  this  catastrophe  as  he  observed  them. 

Ashes  began  to  fall  on  his  ships,  thicker  and  hotter  as  they  approached 
land.  Cinders  and  pumice,  and  also  black  fragments  of  rock  cracked  by 
heat,  fell  around  them.  The  sea  suddenly  shoaled,  and  the  shores  were 
obstructed  by  masses  from  the  mountain.  He  hesitated  awhile  and  thought 
of  going  back  again  ;  but  finally  gave  the  word  to  the  reluctant  helmsman  to 
go  on,  saying,  "  Fortune  favors  the  brave.  Let  us  find  Pomponianus." 
Pomponianus  was  at  Stabia^,  separated  by  the  intervening  bay  (the  sea  comes 
in  here  gradually  in  a  long  inlet  with  curving  shores),  and  although  the  peril 
was  not  near,  yet  as  it  was  in  full  view,  and  as  the  eruption  increased  seemed 
to  be  approaching,  he  had  packed  up  his  things  and  gone  aboard  his  ships 
ready  for  flight,  which  was  prevented,  however,  by  a  contrary  wind. 

My  uncle,  for  whom  the  wind  was  most  favorable,  arrived,  and  did  his  best 
to  remove  their  terrors.    He  embraced  the  frightened  Pomponianus  and  encour- 


52 


ASPECTS    OF  THE  EARTH. 


aged  him.  To  keep  up  their  spirits  by  a  show  of  unconcern,  he  had  a  bath  ; 
and  afterwards  dined,  with  real,  or  what  was  perhaps  as  heroic,  with  assumed 
cheerfulness.  But,  meanwhile,  there  began  to  break  out  from  Vesuvius,  in 
many  spots,  high  and  wide-shooting  flames,  whose  brilliancy  was  heightened 
by  the  darkness  of  approaching  night.  My  uncle  reassured  them  by  assert- 
ing that  these  were  burning  farm-houses  which  had  caught  fire  after  being 


View  in  Pompeii,  looking  Northwest,  showing  the  Unexcavated   Portion  on   the   Right  Hand,  and  in  the  Dis- 
tance the  Present  Cone  of  Vesuvius  ;   on  its  Right  a  Portion  of  Prechristian  Crater-wall. 


deserted  by  the  peasants.  'I'hen  he  turned  in  to  sleep,  and  slept  indeed  the 
most  genuine  slumbers  ;  for  his  breathing,  which  was  always  heavy  and 
noisy,  from  the  full  habit  of  his  body,  was  heard  by  all  who  passed  his  cham- 
ber. But  before  long  the  floor  of  the  court  on  which  his  chamber  opened 
became  so  covered  with  ashes  and  pumice  that  if  he  had  lingered  in  the 
room  he  could  not  have  got  out  at  all.  So  the  servants  woke  him,  and  he 
came  out  and  joined  Pomponianiis  and  others  who  were  watching.  They 
consulted  together  as  to  what  they  should  do  next.     Should   they  stay  in  the 


VOLGA  XOES.  53 

house  or  go  out  of  doors  ?  The  house  was  tottering  with  frequent  and  heavy 
shocks  of  earthquake,  and  seemed  to  go  to  and  fro  as  if  moved  from  its 
foundations.  But  in  the  open  air  there  were  dangers  of  falling  pumice- 
stones,  though,  to  be  sure,  they  were  light  and  porous.  On  the  whole,  to  go 
out  seemed  the  least  of  two  evils.  With  my  uncle  it  was  a  comparison  of 
arguments  that  decided  ;  with  the  others  it  was  a  choice  of  terrors.  So  they 
tied  pillows  on  their  heads  by  way  of  defence  against  falling  bodies,  and 
sallied  out. 

It  was  dawn  elsewhere  ;  but  with  them  it  was  a  blacker  and  denser 
night  than  they  had  ever  seen,  although  torches  and  various  lights  made  it 
less  dreadful.  They  decided  to  take  to  the  shore  and  see  if  the  sea  would 
allow  them  to  embark  ;  but  it  appeared  as  wild  and  appalling  as  ever.  My 
uncle  lay  down  on  a  rug.  He  asked  twice  for  water  and  drank  it.  Then  as 
a  flame  with  a  forerunning  sulphurous  vapor  drove  off  the  others,  the  ser- 
vants roused  him  up.  Leaning  on  two  slaves  he  rose  to  his  feet,  but  imme- 
diately fell  back,  as  I  understand,  choked  by  the  thick  vapors,  and  this  the 
more  easily  that  his  chest  was  naturally  weak,  narrow,  and  generally  inflamed. 
When  day  came  (I  mean  the  third  after  the  last  he  ever  saw)  they  found 
his  body  perfect  and  uninjured,  and  covered  just  as  he  had  been  overtaken. 
He  seemed  by  his  attitude  to  be  rather  asleep  than  dead. 

In  the  mean  time,  my  mother  and  I  at  Misenum — but  this  has  nothing  to 
do  with  my  story.     You  ask  for  nothing  but  the  account  of  his  death.     .     .     . 

[B()t)K  VI.,   20.] 

Gaius  Plinius  sends  to  his  friend  Tacitus  greeting. 

You  say  that  you  are  induced  by  the  letter  I  wrote  to  you,  when  you  asked 
about  my  uncle's  death,  to  desire  to  know  how  I,  who  was  left  at  Misenum, 
bore  the  terrors  and  disasters  of  that  night,  for  I  had  just  entered  on  that  sub- 
ject and  broke  it  off.  "  Although  my  soul  shudders  at  the  memory,  I  will 
begin." 

My  uncle  started  off  and  I  devoted  myself  to  my  literary  task,  for  which  I 
had  remained  behind.  Then  followed  my  bath,  dinner,  and  sleep,  though  this 
was  short  and  disturbed.  There  had  been  already  for  many  days  a  tremor  of 
the  earth,  less  appalling,  however,  in  that  this  is  usual  in  Campania.  But  that 
night  it  was  so  strong  that  things  seemed  not  merely  to  be  shaken,  but  posi- 
tively upset.     My  mother  rushed  into  my  bedroom.     I  was  just  getting  up  to 


54  ASPECTS   OF  THE  EARTH. 

wake  her  if  she  were  asleep.  We  sat  down  in  the  Httle  yard,  which  was 
between  our  house  and  the  sea.  I  do  not  know  whether  to  call  it  courage  or 
foolhardiness  (I  was  only  seventeen),  but  I  sent  for  a  volume  of  Livy,  and, 
quite  at  my  ease,  read  it,  and  even  made  extracts,  as  I  had  already  begun  to 
do.  And  now  a  friend  of  my  uncle's,  recently  arrived  from  Spain,  appeared, 
who,  finding  us  sitting  there  and  me  reading,  scolded  us,  my  mother  for  her 
patience,  and  me  for  my  carelessness  of  danger.  None  the  less  industriously 
I  read  my  book. 

It  was  now  seven  o'clock,  but  the  light  was  still  faint  and  doubtful.  The 
surrounding  buildings  had  been  badly  shaken,  and  though  we  were  in  an  open 
spot,  the  space  was  so  small  that  the  danger  of  a  catastrophe  from  falling  walls 
was  great  and  certain.  Not  till  then  did  we  make  up  our  minds  to  go  from 
the  town.  A  frightened  crowd  went  away  with  us,  and  as  in  all  panics  every- 
body thinks  his  neighbors'  ideas  more  prudent  than  his  own,  so  we  were  pushed 
and  squeezed  in  our  departure  by  a  great  mob  of  imitators. 

When  we  were  free  of  the  buildings  we  stopped.  There  we  saw  many 
wonders  and  endured  many  terrors.  The  vehicles  we  had  ordered  to  be 
brought  out  kept  running  backward  and  forward,  though  on  level  ground  ; 
and  even  when  scotched  with  stones  they  would  not  keep  still.  Besides  this, 
we  saw  the  sea  sucked  down  and,  as  it  were,  driven  back  by  the  earthcjuake. 
There  can  be  no  doubt  that  the  shore  had  advanced  on  the  sea,  and  many 
marine  animals  were  left  high  and  dry.  On  the  other  side  was  a  dark  and 
dreadful  cloud,  which  was  broken  by  zigzag  and  rapidly  vibrating  flashes  of 
fire,  and  yawning  showed  long  shapes  of  flame.  These  were  like  lightnings, 
only  of  greater  extent.  Then  our  friend  from  Spain  attacked  us  more  vigor- 
ously and  earnestly.  "  If  your  brother,  your  uncle,"  said  he,  "is  alive,  he 
wishes  you  to  be  safe  ;  if  not,  he  certainly  would  wish  you  to  survive  him. 
Why,  then,  do  you  delay  your  flight  ?  "  We  said  we  could  not  bring  ourselves 
to  think  of  our  own  safety  while  doubtful  of  his.  So,  without  more  delay,  the 
Spaniard  rushed  off,  taking  himself  out  of  harm's  way  as  fast  as  his  legs  would 
carry  him. 

Pretty  soon  the  cloud  began  to  descend  over  the  earth  and  cover  the  sea. 
It  enfolded  Caprex  and  hid  also  the  promontory  of  Misenum.  Then  my 
mother  began  to  beg  and  beseech  me  to  fly  as  I  could.  I  was  young,  she  said, 
and  she  was  old,  and  too  heavy  to  run,  and  would  not  mind  dying  if  she 
was  not  the  cause  of  my  death.      I  said,  however,  I  would  not  be  saved  with- 


VOLCANOES. 


55 


out  her  ;  I  clasped  her  hand  and  forced  her  to  go,  step  by  step,  with  me.     She 
slowly  obeyed,  reproaching  herself  bitterly  for  delaying  me. 

Ashes  now  fell,  yet  still  in  small  amount.  I  looked  back.  A  thick  mist 
was  close  at  our  heels,  which  followed  us,  spreading  out  over  the  country,  like 
an  inundation.  "  Let  us  turn  out  of  the  road,"  said  I,  "  while  we  can  see,  and 
not  get  trodden  down  in  the  darkness  by  the  crowds  who  are  following,  if  we 


Shows,  on  either  side,  the  depth  of  the  ashcovering.     Vesuvius  in  the  distance. 

fall  in  their  path."  Hardly  had  we  sat  down  when  night  was  over  us — not 
such  a  night  as  when  there  is  no  moon  and  clouds  cover  the  sky,  but  such 
darkness  as  one  finds  in  close-shut  rooms.  One  heard  the  screams  of  women, 
the  fretting  cries  of  babes,  the  shouts  of  men.  Some  called  their  parents,  and 
some  their  children,  and  some  their  spouses,  seeking  to  recognize  them  by 
their  voices.  Some  lamented  their  own  fate,  others  the  fate  of  their  friends. 
Some  were  praying  for  death,  simply  for  fear  of  death.  Many  a  man  raised 
his  hands  in  prayer  to  the  gods  ;  but  more  imagined  that  the  last  eternal  night 


56  ASPECTS   OF  THE  EARTH. 

of  creation  had  come  and  there  were  now  no  gods  more.  There  were  some 
who  increased  our  real  dangers  by  fictitious  terrors.  Some  said  that  part  of 
Misenum  had  sunk,  and  that  another  part  was  on  fire.  They  Hed  ;  but  they 
found  behevers. 

Little  by  little  it  grew  light  again.  We  did  not  think  it  the  light  of  day, 
but  a  proof  that  the  fire  was  coming  nearer.  It  was  indeed  fire,  but  it  stopped 
afar  off ;  and  then  there  was  darkness  again,  and  again  a  rain  of  ashes,  abun- 
dant and  heavy,  and  again  we  rose  and  shook  them  off,  else  we  had  been 
covered  and  even  crushed  by  the  weight.  I  might  boast  of  the  fact  that  not 
a  groan  or  a  cowardly  word  fell  from  me  in  all  the  dreadful  peril,  if  I  had 
not  believed  that  the  world  and  I  were  coming  to  an  end  together.  This 
belief  was  a  wretched  and  yet  a  mighty  comfort  in  this  mortal  struggle.  At 
last  the  murky  vapor  rolled  away,  in  disappearing  smoke  or  fog.  Soon  the  real 
daylight  appeared  ;  the  sun  shone  out,  of  a  lurid  hue,  to  be  sure,  as  in  an 
eclipse.  The  whole  world  which  met  our  frightened  eyes,  was  transformed. 
It  was  covered  with  ashes  white  as  snow. 

We  went  back  to  Misenum  and  refreshed  our  weary  bodies,  and  passed  a 
night  between  hope  and  fear  ;  but  fear  had  the  upper  hand.  The  trembling 
of  the  earth  continued,  and  many,  crazed  by  their  anxiety,  made  ludicrously 
exaggerated  predictions  of  disaster  to  themselves  and  others.  Yet  even  then, 
though  we  had  been  through  such  peril  and  were  still  surrounded  by  it,  we 
had  no  thought  of  going  away  till  we  had  news  of  my  uncle.     .     .     . 

It  is  evident  that  this  eruption  produced  great  changes  in 
the  surface  of  all  the  country  about  Vesuvius.  Although  no 
lava-streams  flowed  from  the  crater,  for  the  reason,  as  we  shall 
hereafter  see,  that  the  eruption  was  so  violent  as  to  prevent 
their  formation,  the  quantity  of  molten  rocky  matter  which 
was  blown  into  fragments  and  fell  mainly  in  the  form  of  dust 
upon  the  surface  of  the  earth  about  the  crater  was  enormous. 
For  a  distance  of  several  miles  from  the  vent,  this  accumula- 
tion seems  to  have  attained  the  depth  of  ten  to  thirty  or  more 
feet.  Owine  to  the  extreme  lii^^htness  of  this  dust,  which  is 
pumiceous,  or  filled  with   air-bubbles,  the  greater  part  of  the 


VOLCANOES.  57 

deposit  has  probably  been  washed  away  by  the  rain,  as  have 
the  lesser  ash-showers  of  later  years.  At  the  close  of  the 
eruption  of  Pliny,  this  dust  probably  covered  the  ground  to 
a  far  greater  depth  than  Is  indicated  by  the  scanty  remains 
of  the  crreat  shower  which  still  exist  on  the  surface.  On  no 
other  supposition  can  we  account  for  the  abandonment  of  the 
two  cities  of  Pompeii  and  Herculaneum,  which  were  so  far 
lost  that  no  tradition  as  to  their  position  remained.  Both  of 
these  cities  were  probably  stripped  of  their  more  precious 
treasures  before  they  were  covered  with  the  ash,  and  the  mud 
which  was  formed  of  it  by  the  torrential  rains  ;  still  so  much 
that  was  valuable  was  left  behind,  that  we  can  hardly  conceive 
how  the  dispossessed  people  should  have  failed  to  dig  for  the 
treasures,  unless  they  were  deterred  by  a  thicker  sheet  of 
debris  than  now  remains  upon  Pompeii. 

At  the  close  of  this  eruption  the  surface  of  the  country 
immediately  about  Vesuvius  must  have  been  a  waste  of  ashes. 
Besides  the  two  important  towns  of  Herculaneum  and  Pom- 
peii, there  were,  it  may  be,  scores  of  villages  which  were 
burled  In  the  same  way.  It  is  not  likely  that  the  loss  of  life 
in  this  catastrophe  was  very  great.  It  was  some  hours  before 
the  eruption  became  of  fatal  violence,  and  nearly  all  the 
inhabitants,  save  the  sick  and  prisoners,  found  safety  in  flight. 
Of  the  hundred  or  so  skeletons  which  have  been  found  in  the 
excavation  at  Pompeii,  some  appear  to  be  the  remains  of  sol- 
diers, who,  receiving  no  orders  to  withdraw,  met  death  In  their 
appointed  places.  Occasionally  as  the  explorers  are  remov- 
ing the  firmly  cemented  ash  from  the  cellars  of  a  house,  their 
picks  penetrate  a  cavity.  Experience  has  shown  that  these 
spaces  are  generally  moulds  which  the  wet  ashes  formed  about 
a  prostrate  human  body.      By  pouring  plaster-of-Paris  into  the 


58  ASPECTS   OF  THE  EARTH. 

empty  places,  it  has  been   found  possible  to  obtain  accurate 
casts  of  the  long-vanished  forms. 

The  eruption  of  the  year  79  was  followed,  as  is  usual  after 
great  eruptions,  by  a  long  period  of  repose.  The  next  out- 
break of  the  volcano  was  in  the  year  203,  and  appears  to 
have  been  of  moderate  violence.      After  another  equally  long 


A  Lava-stfeam  Overwhelming  a  Town  on  the  West  Side  of  Vesuvius. 

pause,  in  472  there  was  an  extremely  violent  eruption,  which 
is  reported  to  have  scattered  ashes  over  nearly  all  Europe, 
and  so  darkened  the  sky  at  Constantinople,  about  eight  hun- 
dred mijes  away,  that  the  Emperor  Leo  iled  from  the  city, 
and  for  a  long  period  thereafter  the  deliverance  of  the  town 
was  celebrated  by  an  annual  festival.       Thence  to  the  year 


VOLCAXOES.  59 

1036  of  our  era  we  have  records  of  occasional  slight  erup- 
tions, but,  as  the  reader  knows,  this  was  the  night-time  of 
history,  and  the  chronicles  are  very  imperfect.  In  1036  it 
seems  tolerably  clear,  from  an  ancient  itinerary,  that  lava 
flowed  from  the  cone  to  the  sea.  This  appears  to  have  been 
the  first  eruption  during  the  historic  period  in  which  lava 
flowed  from  Vesuvius,  though  in  the  prehistoric  period  of  the 
mountain's  acti\ity  it  was  abundantly  produced. 

From  this  eruption  onward  to  modern  times  \ve  have  an 
excellent  catalogue  of  the  eruptions  of  both  Vesuvius  and 
i^tna,  which,  curiously  enough,  we  owe  in  good  part  to  the 
superstitious  notion  that  the  outbreaks  may  be  stopped  by 
the  intercession  of  the  patron  saints  of  the  country.  When- 
ever an  eruption  occurs  the  priests  who  guard  the  relics  of 
St.  Januarius,  in  Naples,  or  of  St.  Agatha,  in  Sicily,  address 
these  patrons  of  their  respective  cities  through  their  relics, 
vestments,  or  images.  If  the  eruption  speedily  diminishes  in 
violence,  as  from  the  nature  of  its  action  it  must  always  do, 
the  amendment  Is  attributed  to  the  influence  of  the  saintly 
power,  and  the  fact,  with  date  and  circumstance,  is  a  matter  of 
careful  record.*  Thus  science  has  come  to  owe  a  consider- 
able debt  to  superstition.  Although  this  picturesque  relation 
adds  a  certain  interest  to  the  chronicles  of  the  eruptions  of 
Vesuvius,  we  need  not  weary  the  reader  with  them,  but  sum 
up  the  record  in  brief.  In  short,  the  story  is  that  from  1036 
to  I  500  there  were  five  eruptions,  or  about  one  each  century, 
and  none  of  them  of  great  violence.  It  seems,  indeed,  likely 
that  from    11 39  to  1631  there  were   at   most  slight  threats  of 

*See  "Vesuvius"  fpage  45),  by  John  Phillips.  Clarendon  Press,  Oxford,  1859. 
From  this  valuable  work  I  have  condensed  the  above  statements  concerning  this 
volcano. 


6o  ASPECTS    OF  THE  EARTH. 

activity,  and  that  the  internal  pressure  was  not  relieved  until 
the  great  explosion  of  the  last-named  year. 

The  eruption  of  1631  was,  next  after  that  of  79,  the  most 
violent  explosion  which  has  taken  place  from  Vesuvius.  Like 
the  eruption  \n  which  Pliny  met  his  death,  the  disturbance 
was  ushered  in  by  a  succession  of  earthquake  shocks.  These 
shocks,  due  doubtless  to  the  struggle  of  the  imprisoned 
gases  with  the  barriers  which  the  earth  interposed,  grew  more 
and  more  violent,  until,  on  December  16,  the  outbreak  began 
suddenly  and  with  extreme  fury.  Unlike  most  eruptions 
from  this  and  other  craters,  where  the  flow  of  liquid  rock 
usually  begins  some  time  after  the  gases  break  forth,  a  great 
tide  of  lava  at  once  burst  forth  from  the  side  of  the  cone,  at 
some  distance  from  the  summit  of  the  crater.  The  streams 
rushed  forth  from  a  number  of  points  along  the  southwest 
slope  of  the  mountain,  at  a  height  of  about  three  thousand 
feet  above  the  sea,  and  swept  down  toward  the  shore  of  the 
bay.  Although  a  large  i)art  of  this  lava  remained  in  the 
depressions  in  the  flanks  of  the  mountain,  a  dozen  or  more  of 
the  streams  which  diverged  from  the  great  sheet  attained  the 
sea  along  a  length  of  seven  and  a  half  miles  of  the  shore. 
Then,  as  now,  the  coast  was  bordered  by  an  almost  continu- 
ous line  of  populous  towns.  Although  the  inhabitants  had 
fied  in  great  numbers,  moved  by  the  fear  with  which  the 
earthquakes  and  roarings  from  the  mountain  inspired  them, 
the  lava-flow  came  so  suddenly  that  eighteen  thousand  per- 
sons perished  in  the  towns  of  Resina,  Torre  del  Greco,  and 
Granatello,  which  were  overwhelmed  by  the  streams.  The 
ash,  or  flnely  divided  lava,  was  blown  forth  in  prodigious 
quantities,  once  again  darkening  the  skies  as  far  to  the  east 
as    Constantinople.       The    rain     which    fell  from     the    cloud 


J'OLCAXOES. 


6i 


which  hung  over  all  the  region  about  the  mountain  was 
torrential  ;  mingled  with  the  fine  dust,  it  produced  vast  inun- 
dations of  mud,  which  swept  over  the  fields  and  villages, 
producing  destruction  more  wide-spread,  if  less  disastrous  to 
life,  than    the    streams    of    fiery  lava.      In  this,  as  in  all    the 


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Vesuvius;  Near  View  ofthe  Small  Inner  Cone  of  the  Crater,  showing  Recent  Undecayed  Lava  on  wnicn  Rests 

the  Ash-heap  of  the  Cone. 

great  eruptions,  the  lightning  from  the  clouds  was  extremely 
violent  and  caused  much  loss  of  life. 

From  the  time  of  this  disaster  down  to  the  present  day 
the  eruptions  have  been  more  frequent  than  in  any  other  part 
of  the  volcano's  history.  Rarely  have  twenty  years  passed 
without  an  outbreak  of  considerable  violence,  though  none  of 
them  have  attained  to  the  appalling  fury  of  the  first  historic 
outbreak    or  that    of    1631.       Near    four-score  eruptions    are 


62  ASPECTS    OF  THE  EARTH. 

chronicled  in  this  period  of  about  two  and  a  half  centuries  ; 
nearly  all  of  them  have  been  of  moderate  intensity,  but  have 
led  to  a  singularly  large  extrusion  of  lavas.  It  is  evident  that 
the  channels  which  lead  to  the  rents  of  the  volcano  are  now 
troro-ed  with  fluid  lava  ;  wherever  the  pressure  of  the  impris- 
oned  gases  becomes  strong  enough  this  lava  is  forced  up 
into  the  crater,  by  its  weight  rends  open  the  walls  of  inco- 
herent cinders,  and  escapes  upon  the  steep  slopes  of  the 
cone. 

Many  of  these  outbreaks  are  of  very  slight  energy.  It 
was  the  present  writer's  good  fortune  to  obtain  an  unusually 
near  view  of  the  beautiful  little  eruption  of  the  winter  of  1882, 
which  afforded  a  singularly  good  opportunity  for  watching  the 
essential  processes  of  volcanic  explosions  with  little  danger. 
At  this  time,  from  the  slight  violence  of  the  outbreak,  the 
crater  was  reduced  to  a  small  depression  near  the  summit 
of  the  cone,  which  had  a  diameter  of  not  over  six  hundred 
feet  and  a  depth  of  about  one  hundred  feet.  Taking  advan- 
tao-e  of  a  strong  gale  from  the  north,  the  well-known  tranwn- 
taiia  of  Ital\-,  it  was  possible  to  creep  up  to  the  very  edge  of 
this  crater  and  look  down  upon  the  surface  of  the  boiling 
lava,  from  which  the  gases  were  breaking  forth.  Although 
the  pit  was  from  time  to  time  filled  with  whirling  vapor,  the 
favoring  wind  often  swept  it  away  so  that  for  a  few  seconds 
it  was  possible  to  see  every  feature  of  the  terrifying  scene. 
Several  times  a  minute  the  surface  of  the  tossed  lava  was 
rent  by  a  violent  explosion  of  gases,  which  appeared  to  hurl 
the  whole  mass  of  tUiid  rock  into  the  air.  The  ascending 
column  of  vapor  and  lava  fragments  rose  as  a  shaft  to  the 
height  of  several  hundred  feet.  Many  of  the  masses,  which 
seem(;d   to   rise  with    the    ease  of   bubbles,  were   some   feet   in 


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VOLCANOES.  63 

diameter,  and  made  a  great  din  as  they  crushed  down  upon 
the  surface  on  the  southward  side  of  the  crater.  They  often 
could  be  seen  to  fly  into  fragments  as  they  ascended.  At  the 
moment  of  the  explosion  the  escaping  gases  appeared  trans- 
parent, a  few  score  feet  above  the  point  of  escape  the  ejected 
column  became  of  a  steel-gray  color,  and  a  little  higher  it 
changed  to  the  characteristic  hue  of  steam.  That  it  was 
steam  slightly  mixed  with  other  gases  was  evident  wherever 
in  its  whirling  movements  the  vaporous  column  swept  around 
the  point  of  observation.  The  curious  "  washing-day "  odor 
of  steam  was  perfectly  apparent,  together  with  a  pungent 
sense  of  sulphurous  fumes  suggestive  of  an  infernal  laundry. 

Although  the  heat  at  the  moment  of  explosion  was  great, 
it  was  possible,  with  the  shelter  to  the  face  secured  by  an 
extemporized  mask,  to  avoid  any  serious  consequences  from 
it,  and  even  to  make  some  rather  rude  and  unsatisfactory  dia- 
grams of  the  scene.  The  principal  obstacle  to  observation 
arose  from  the  violence  of  the  shocks  given  to  the  cone  and 
propagated  through  the  air  by  the  explosions,  which  made  it 
extremely  difficult  to  fix  the  attention  on  the  phenomena. 
The  earthquakes  attending  each  explosion  were  almost  strong 
enough  to  shake  one  from  the  s^round,  and  the  blow  received 
throuorh  the  air  was  like  that  which  those  familiar  with  mines 
have  felt  w^hen  a  heavy  charge  of  gunpowder  or  dynamite 
is  exploded.  The  sensation  is  such  as  might  come  from 
being  violently  struck  by  a  feather  bed  ;  not  dangerous,  but 
extremely  disorganizing  to  the  wits.  After  about  fifteen 
minutes  of  observation  a  slight  change  of  the  wind  allowed 
the  descending  masses  to  fall  so  near  the  point  of  view 
that  it  was  necessary  to  hurry  away. 

As  if  to  complete  the   illustration  of  volcanic  phenomena 


64  ASPECTS    OF  THE  EARTH. 

which  this  little  outbreak  afforded,  there  was  a  small  rivulet 
of  lava  pouring  from  the  low  wall  of  cinders  on  one  side  of 
the  cone  and  flowing  quietly  down  the  slope.  It  was  not 
much  larger  than  the  stream  of  liquid  iron  which  flows  from 
an  iron-furnace  to  the  moulds  which  await  it,  but  in  the 
motion  all  the  essential  features  of  the  greatest  of  these  fiery 
torrents  could  be  seen.  The  surface  of  the  fluid,  cooled  in 
the  air,  slowly  hardened  into  a  viscid  scum.  This  scum,  urged 
forward  by  the  swifter  movement  of  the  more  fluid  matter 
below,  was  wrinkled  as  is  the  cream  on  a  pan  of  milk  when  it 
is  slowly  poured  over  the  edge  of  the  vessel. 

A  tiny  eruption  such  as  this  can  be  transformed  into  those 
of  the  greatest  energy  by  simply  increasing  the  volume  of  the 
discharging  gases.  We  have  only  to  conceive  the  ascending 
column  of  intensely  heated  steam,  in  place  of  breaking  out  In 
the  separate  cannon-like  explosions,  pouring  forth  in  a  con- 
tinuous rush  and  mounting  to  the  height  of  several  miles 
above  the  vent ;  the  increased  force  of  the  outbreak  blowing 
away  the  summit  of  the  cone,  enlarging  the  crater  until  it  was 
perhaps  a  mile  in  diameter  ;  the  steam  imprisoned  in  the  frag- 
ments of  lava  tossed  up  by  the  explosion  expanding  with  great 
energy,  not  only  rupturing  the  blocks,  but  rending  them  into 
powder,  and  the  rivulet  of  lava  magnified  to  a  torrent  such 
as  so  often  sweeps  clown  the  flanks  of  the  mountain.  Thus, 
by  a  change  in  the  magnitude  of  the  action  alone,  we  pass 
from  the  most  trifling  to  the  greatest  eruptions. 

This  glance  at  the  history  and  structure  of  Vesuvius  serves 
to  give  us  a  general  notion  of  eruptions  ;  we  see  that  they 
are  essentially  jets  of  extremely  heated  steam,  and  that  the 
ashes  and  lava,  though  they  are  the  only  permanent  remains 
of  the  successive  explosions,   are  by  far  the  least    important 


VOLCANOES. 


65 


element  of  the  matter  cast  forth  during  an  eruption.  It 
seems  probable  that  if  we  could  gather  again  all  the  water 
which  in  the  form  of  steam  has  poured  from  Vesuvius  since 
the  cone  beean  to  form,  we  should  find  that  it  amounted  in 
mass  to  several  times  as  much  as  all  the  ash  and  lava  which 


Vesuvius,  looking  East  from  the  "  Observatory,"  1880,  showing   Vent-cone  and   Old    Eroded    Pedestal  of  Lava 

and  Ash. 

The  dark  line  on  the  right  of  the  cone  is  the  railway  up  the  mountain. 

forms  the  cone.  This  water  falls  in  torrential  rains  in  the 
region  about  the  crater,  or  drifts  away  in  clouds  to  other 
countries,  and  so  leaves  no  sign  except  in  the  furrowed  sides 
of  the  volcano,  which  are  deeply  eroded  by  the  floods  that 
attend  the  greater  eruptions.  We  may  compare  the  explosion 
of  a  volcano  to  the  action  of  a  bursting  boiler,  when  in  a 
moment  the   rupturing  agent   disappears   in   the   air,  leaving 


66  ASPECTS   OF   THE  EARTH. 

only  the  fragments  of  the  vessel  which  contained  it  and  which 
it  has  torn  to  pieces. 

A  large  part  of  the  materials  thrown  out  by  a  volcano  does 
not  fall  upon  the  cone  ;  in  most  of  the  eruptions  of  Vesuvius 
the  dust  has  been  the  largest  part  of  the  solid  matter  cast 
forth,  the  lava  perhaps  not  amounting,  on  the  average,  to 
as  much  as  one-fiftieth  of  the  mass  of  rock-material  ejected. 
The  coarser  part  of  this  dust  falls  in  the  region  near  the  cone, 
but  a  large  share  of  it  drifts  to  great  distances,  to  darken  the 
skies,  it  may  be,  a  thousand  miles  away.  During  several  of 
the  great  eruptions  of  Vesuvius  the  dust  which  fell  within  ten 
miles  of  the  crater  formed  a  stratum  averaeine  more  than  a 
foot  in  depth,  greatly  exceeding  in  volume  the  ejected  lava  ; 
still  it  seems  likely  that  by  far  the  larger  part  of  this  dust  did 
not  fall  near  the  crater,  but  was  borne  by  the  winds  far  ana 
wide  over  land  and  sea. 

We  may  here  remark  the  fact  that  the  quantity  of  dust 
ejected  during  a  volcanic  explosion  varies  very  greatly,  not  only 
in  different  volcanoes,  but  in  various  eruptions  from  the  same 
cone.  The  quantity  of  this  comminuted  material  as  well  as 
its  fineness  of  division  appears  to  depend  upon  the  energy  of 
the  explosion.  Where  the  movement  of  the  materials  upward 
through  the  throat  of  the  volcano  is  relatively  slow,  the  steam 
produced  by  the  water  originally  imprisoned  in  the  confined 
interspaces  of  stone  boils  out  of  the  lava  and  escapes  in  clouds 
of  vapor.  Where  the  uprush  of  the  lava  is  swift,  each  little 
vesicle  of  water  swiftly  expands  and  blows  the  rock  into 
extremely  fine  bits.  The  nature  of  the  action  can  be  made 
more  readily  understood  by  a  reference  to  an  interesting 
mechanical  contrivance  which  was  invented  to  make  paper 
pulp   froni   the   stems   of  the   common   cane.      In   this   contriv- 


VOLCANOES. 


67 


ailce  the  hard  woody  matter  of  the  reeds  was  enclosed  in  a 
strong  iron  cylinder    shaped  much  like  an  ordinary  cannon. 


Showing  Volcanic  Tufa  of  Naples,  in  which  Subterranean  Dwellings  have  been  Excavated. 

Deposit  formed  of  volcanic  ash  laid  down  on  the  sea-floor  during  prehistoric  eruptions  in  the  Vesu- 
vian  district. 


The  vessel  had  a  lid  which  closed  the  mouth  of  the  chamber. 
Into  the  hollow  of  the  gun   the  cane  was  charged,  along  with 


68  ASPECTS   OF   THE  EARTH. 

enoLitrh  water  to  enclose  the  mass  of  stems.     The  entrance  to 
the  chamber  being  closed  by  the  lid,  the  gun  was  then  heated 
to    a    temperature    far    above    the     boiling    point    of   water. 
Restrained    by  the    pressure,    the    water    did    not    pass    into, 
steam,  but  the  fluid  within  the  interspaces  of  the  woody  matter., 
was  brought  to  a  temperature  at  which  it  would  immediately 
pass  into  the  state   of  vapor    as    soon    as    the    pressure  was 
removed.      After  some  hours  of  heating,   the  lid  of  the  gun 
was  suddenly  opened,  whereupon  all  the  water  passed  into  the 
state   of  steam,  the  cane  was  blown   into  bits,   and  the  mass 
driven  forth  from  the  gun.     This  process,  whereby  the  woody  , 
fibre  was  finely  divided  through  the  expansion  of  the  contained 
steam,  is  exactly  the  same  as  that  by  which  the  imprisoned 
water  in  the  lava  disrupts  the  stone  when  the  mass  escapes 
from  the  pressure  of  the  earth  into  the  open  air. 

After  the    reader  has  conceived  the  magnitude  and  con- 
tinuity of  the  Vesuvian  eruptions,  it   is  well   to   consider  that  ; 
this  vent  is  really  a  very  small  affair,  not  deserving  to  rank  as-  ' 
more  than  a  third-rate  volcano,  if  we  determine   the  order  of  '• 
importance  by  the  size  of  the  cone,  the  diameter  of  the  vol-  .': 
canic  tube,   or  the   velocity  of  the   eruptions.      The  family  of   ■ 
Italian    volcanoes    includes  at   least   three   other  vents  which^*, 
have,  or  have  had  in  their  period  of  activity,  a  larger  measure^;;! 
of  dignity  than  the  Vesuvian  cone.     yEtna  has  at  least  twenty"*  : 
times  the  bulk,   and  presents  to  us    phenomena  of  Vesuvius 
exhibited  on  a  far  greater  scale.      Among  the   numerous  dor- 
mant or  extinct  volcanoes  which  lie  along  the  shore  between 
Naples  and  Southern  Tuscany,  those  of  Bracciano  and   Bol^v: 
sena,  whose  vast  craters  are  now  occupied  by  lakes,  were  in  - 
their  time  far  more  majestic  than  Vesuvius.     The  crater  of 
Bolsena  now  affords  a  basin  for  a  lake  havino-  an  area  of  about " 


VOLCANOES. 


69 


forty  square  miles,  and  yet  the  whole  of  its  vast  expanse  is 
not  completely  occupied  by  the  sheet  of  water.  It  is  doubtful 
if  the  area  of  the  Vesuvian  crater  was  ever  six  square  miles. 
That  of  Bracciano  is  smaller  than   Bolsena,  but  still  several 


Crater,  Lakes  of  the  Seven  Cities,  St.  Michael's,  Azores. 
There  are  two  of  the  craters  united  by  the  breaking  down  of  a  part  of  the  bordering  walls. 

times  as  large  as  the  Vesuvian  crater.  These  two  volcanoes- 
of  Bolsena  and  Bracciano  were  giants  in  their  youth,  but  they 
came  to  an  untimely  end.  Their  subterranean  fires  were 
extinguished  before  they  had  time  to  construct  cones  at  all 
proportionate  to  their  vast  orifices. 


JO  ASPECTS   OF  THE  EARTH. 

Although  the  total  number  of  volcanoes,  active  and  extinct, 
amounts,  in  Europe,  to  several  hundred,  including  those  of 
Central  France  and  Germany  and  the  peripheral  cones  of 
^tna,  we  must  go  beyond  the  bounds  of  that  continent  to 
find  instances  of  eruptions  of  the  tirst  order.  The  noblest 
and  most  characteristic  volcanoes,  whether  we  class  them  by 
the  energy  of  their  explosions  or  the  volume  of  their  ejec- 
tions, are  found  in  Iceland  and  in  the  Malayan  Archipelago. 
In  Iceland  the  volcano  of  Skaptar,  in  the  single  eruption  of 
1783,  poured  out  a  tide  of  lava  exceeding  in  bulk  all  that  has 
flowed  from  Vesuvius  and  yEtna  combined  since  the  eruption 
of  Pliny.  It  has  indeed  been  computed  to  be  greater  than 
the  mass  of  Mont  Blanc.  The  gas-eruption  which  attended 
this  molten  tide  was  proportionally  great  ;  the  clouds  of  fine 
cinders  floated  over  Europe  and  so  darkened  the  sky  as  to 
occasion  fears  of  some  great  calamity.  Although  Iceland 
is  a  thinly  peopled  country,  this  catastrophe  was  extremely 
destructive  to  human  life  ;  nearly  a  fifth  of  the  population 
perished  in  the  villages  which  were  overwhelmed  by  the  erup- 
tion, from  the  famine  which  came  from  the  loss  of  the  year's 
crops,  and  the  frightening  of  the  fish  from  the  neighboring 
sea. 

The  whole  of  the  island  of  Iceland  is  composed  of  volcanic 
matter,  which  has  been  cast  forth  in  a  succession  of  eruptions 
within  modern  geological  periods.  There  have  probably  been 
scores  if  not  hundreds  of  eruptions  in  Iceland  as  great  as  that 
of  1783.  Again  and  again  we  must  conceive  these  explosions 
to  have  poured  their  dust  and  vapor  into  the  atmosphere, 
scattering  the  stony  waste  far  and  wide  over  the  bottom  of 
the  North  Atlantic  and  the  lands  which  border  it.  The  erup- 
tion  of   1783  was  the  greatest  in   the  history  of  this  volcanic 


VOLCANOES.  7 1 

district,  but  such  explosions  have  probably  been  frequent  in 
the  development  of  the  island. 

It  is  interesting  to  note  the  effect  of  this  eruption  on  the 
minds  of  the  people  in  the  eighteenth  century  and  to  contrast 
it  with  that  arising  from  the  great  eruption  of  Krakatoa  here- 
after to  be  described.  The  outbreak  of  Skaptar  in  1 783,  as 
well  as  that  of  Krakatoa  in  1883,  cast  a  great  deal  of  watery 
vapor  and  of  finely  divided  ash  into  the  atmosphere.  In  both 
cases  the  skies  were  much  clouded  by  the  emanations,  which 
produced  a  singular  redness  at  dawn  and  eve.  A  century 
ago  this  change  in  the  aspect  of  the  heavens  produced  a  pro- 
found and  wide-spread  fear,  a  sense  of  impending  calamity ; 
even  the  poet  Cowper,  a  well-informed  gentleman  of  his  time, 
held  this  phenomenon  to  indicate  in  some  way  the  wrath  of 
God  or  to  be  a  presage  of  ills  to  come.  Science  had  no 
place  in  the  public  discussion  of  these  facts  in  the  eighteenth 
century  ;  on  the  other  hand,  in  1883  the  much  more  wide- 
spread and  conspicuous  disturbance  of  the  air  due  to  the 
eruption  of  Krakatoa  became  at  once  the  matter  of  scientific 
inquiry.  The  public  mind  was  in  no  way  made  anxious 
about  the  singular  condition  of  the  heavens ;  science  came  in 
to  explain  the  phenomenon,  and  the  reasoning  was  immedi- 
ately grasped  by  the  public.  In  this  contrast  we  see  clearly 
the  effects  of  advancing  natural  learning  and  the  consequent 
change  in  the  attitude  of  men  toward  natural  phenomena. 

The  thousand  years  of  struggle  which  the  Icelanders  have 
had  with  polar  cold  and  central  fire  is  one  of  the  most  pathetic 
incidents  in  the  history  of  our  race.  Almost  every  genera- 
tion on  that  island  has  borne  a  heavy  burden  from  earth- 
quake-shocks or  volcanic  explosions,  and  yet  this  people  have 
managed,  by  labor  and  thrift,  to  develop  and  maintain  a  well- 


72  ASPECTS    OF   THE   EAR'IH. 

ordered  civilization.  For  centuries  the  social  order  has  been 
more  secure,  education  more  general,  and  the  moral  quality 
purer  than  in  the  happier  parts  of  the  world.  Everywhere 
else  save  in  this  marvellous  island  we  find  that  the  effect  of  a 
hopeless  contest  with  physical  ills  is  to  degrade  man  in  spirit. 
Owing  to  the  rapid  extension  of  the  Darwinian  hypothesis^ 
we  find  many  historians  disposed  to  explain  the  intellectual 
and  moral  conditions  of  people  by  means  of  their  physical 
surroundings.  There  are  those  who  are  disposed  to  account 
for  the  culture  of  Greece,  in  part  at  least,  through  the  influ- 
ence of  its  benign  climate,  and  to  explain  the  retardation  of 
tropical  peoples  or  those  near  the  poles  by  the  burden  which 
nature  imposes  upon  men  in  such  situations.  The  intellectual 
history  of  Iceland  when  properly  considered  must  give  us 
pause  in  such  speculations,  for  it  shows  us  that  the  innate 
qualities  of  the  people  may  go  far  to  exempt  them  from  the 
influence  of  their  surroundings.  It  Is  true  that  Iceland  has 
developed  no  art,  but  in  every  other  feature  of  its  intellectual 
development  there  is  much  analogy  between  the  folk  of  this 
afll'cted  land  and  those  of  the  fortunate  region  of  Hellas : 
an  intense  political  life,  a  strong  historic  spirit,  and  poetic 
motive,  less  wide-ranging  but  fit  to  be  compared  in  intensity 
with  that  of  Greece,  mark  their  intellectual  development. 
There  can  be  no  doubt  that  men  are  greatly  influenced  by 
their  physical  surroundings,  but  the  fact  that  there  is  a  limi- 
tation in  the  measure  of  this  influence  is  clearly  shown  by 
Icelandic  history. 

Although  Iceland's  Skaptar  is  a  great  volcano,  and  as  a 
lava-producer  has  perhaps  the  first  place  among  volcanoes,  it 
is  in  the  region  about  the  Pacific  Ocean  that  we  find  the  kings 
of  this  race  of  (dants.     Around  the  shores  of  this  ereat  area  of 


VOLCANOES. 


73 


waters  we  have  a  singularly  continuous  line  of  volcanic  vents. 
Counting  only  those  which  have  been  in  activity  since  the 
beginning  of  the  present  geological  period,  the  aggregate 
probably  amounts  to  many  hundreds.  Although  the  volcanic 
enercries  are,  or  have  recently  been,  violent  in  all  parts  of  this 
vast  field,  they  exhibit  their  maximum  energy  in   the  central 


Volcanic  Cone,  Sandwich  Islands,  showing   the    Aspect  of   Crater-walls  and    Floor  after  the   Surface   has   been 

Covered  by  Vegetation. 

part  of  the  great  Malayan  Archipelago.  This  region  has 
been  well  termed  a  "  rookery  of  volcanoes."  Not  only  are 
great  cones  more  numerous  in  this  field  than  in  any  other 
equal  area,  but  we  have  had  there  the  greatest  eruptions  of 
which  we  have  any  historical  record.  We  can  note  only  a 
few  of  these  great  explosions. 

In  1772  Papandayang,  a  great  volcano  over  nine  thousand 


74  ASPAC7S   OF  THE  EARTH. 

feet  high,  broke  out  with  such  violence  that  the  upper  part 
of  the  cone  for  a  height  of  four  thousand  feet  was  tossed  into 
the  air,  and,  together  with  a  prodigious  amount  of  ashes  dis- 
charged by  the  eruption,  overwhelmed  forty  villages.  In  1822 
Sumbowa,  on  an  island  a  little  to  the  east  of  Java,  was  the  seat 
of  a  yet  more  powerful  eruption.  As  in  the  other  great  explo- 
sions of  this  region,  the  sound  was  heard  a  surprising  distance, 
being  audible  in  Sumatra,  nine  hundred  and  seventy  geograph- 
ical miles  to  the  west,  and  at  Ternate,  seven  hundred  and 
twenty  miles  to  the  east.  This  is  as  if  a  volcano  at  Chicago 
should  make  its  explosions  heard  by  the  people  in  Boston 
and  Omaha.  The  fall  of  ash  and  pumice  was  enormous  ;  it 
crushed  buildings  more  than  forty  miles  from  the  crater. 
Whirlwinds,  caused  by  the  atmospheric  disturbance  common 
in  all  great  eruptions,  rent  the  forests  from  their  roots,  and 
did  much  to  complete  the  catastrophe  which  reduced  a  popu- 
lous and  fertile  region  to  a  desert.  Of  twelve  thousand  people 
in  the  province  of  Tomboro,  in  which  the  crater  is  situated, 
but  twenty-six  escaped  alive.  In  1822  also  Galongoon,  a 
crater  never  before  known  to  have  been  in  activity,  exploded 
with  extreme  violence,  and  in  a  period  of  four  hours  covered 
the  country  about  It  with  a  thick  coating  of  ashes  and  hot 
mud,  destroying  one  hundred  and  forty  villages,  with  a  loss  of 
four  thousand  lives.  This  coating  of  mud  was  so  thick  that 
for  the  distance  of  twenty-four  miles  on  one  side  of  the  moun- 
tain there  were  no  visible  remains  of  the  numerous  settle- 
ments which  had  existed  there  before  the  eruption  began. 

In  1883  a  century  of  gigantic  eruptions  was  completed  by 
the  outbreak  of  Krakatoa,  by  far  the  greatest  explosion  of 
which  we  have  any  account.  Krakatoa  is  a  small  island  lying 
between  the  greater  masses  of  Java  on  the  east  and   Sumatra 


VOLCANOES.  75 

on  the  west.  Although  manifestly  a  volcano,  it  is  likely  that 
it  had  never  within  historic  times  been  in  eruption  until  May 
23,  1883.  At  that  time  it  was  the  seat  of  an  outbreak  which 
was  considered  trifling,  only  adding  one  more  to  the  many 
points  of  modern  volcanic  activity  in  that  region.  The  erup- 
tion was  soon  over,  and  on  the  27th  of  the  month  many 
observers  visited  the  mountain  to  note  the  changes  which  it 
had  brought  about.  For  three  months  it  seemed  absolutely 
quiet ;  but  in  August  of  the  same  year,  with  little  preliminary 
commotion,  a  memorable  outbreak  occurred.  Nearly  the 
whole  of  the  original  island  was  blown  away  down  to  below 
the  sea-level,  probably  at  the  first  discharges  of  the  gases,  so 
that  the  greater  part  of  the  eruption  took  place  from  the  floor 
of  the  sea.  The  violent  boundings  of  this  floor  created  vast 
waves  in  the  ocean,  which  rose  to  the  height  of  fifty  or  sixty 
feet  along  the  populous  shores  of  the  neighboring  islands  of 
Sumatra  and  Java,  sweeping  away  villages  and  plantations,  and 
killing  over  thirty  thousand  people.  Thence,  with  diminish- 
ing height,  these  waves  rolled  onward  like  the  tides  until  they 
were  felt  in  the  Northern  Atlantic  and  along  nearly  the  whole 
of  the  Pacific  shore. 

The  movements  which  this  shock  impressed  on  the  atmos- 
phere were  even  more  remarkable  than  those  which  it  gave  to 
the  sea.  The  sounds  of  the  explosions  were  heard  for  double 
the  distance  to  which  we  have  any  record  of  their  having  been 
audible  in  previous  eruptions.  If  an  eruption  of  Skaptar  in 
Iceland  should  be  audible  at  once  along  our  great  lakes  and 
upon  the  Mediterranean,  we  should  have  a  case  of  sound- 
transmission  comparable  to  that  in  Krakatoa  in  August,  1883. 
The  waves  of  the  air  caused  by  the  sudden  pressure  of  the 
escaping  gases  rolled  around  the  earth,  twice  girdling  its  cir- 


76 


ASPECTS    OF  THE  EARTH. 


CLimference.  Besides  the  enormous  mass  of  dust  which  fell 
upon  land  and  sea  within  a  few  hundred  miles  of  the  point  of 
explosion,  which  probably  amounted  in  bulk  to  as  much  as 
twelve  cubic  miles,  an  unknown  amount  of  the  more  finely 
comminuted  rock  remained   for  a  long  time  suspended  in  the 


A  Crater  in  the  Sandwich  Islands  at  the  Close  of  Eruption  ;   Showing  Lava-terraces  and  Stratified  Nature 

of  Cone. 

atmosphere  and  was  floated  over  all  parts  of  the  earth's  sur- 
face, q-iving  to  the  sky  at  morninor  and  evening  the  memor- 
able rudd)'  glow  it  presented  in  the  two  years  following  the 
eruption.  The  amount  of  this  widely  scattered  matter  cannot 
be  accurately  com})uted,  but  it  possibly  exceeded  in  volume 
that  which  fell  about  the  crater. 


JVLCANOFS.  yy 

The  foregoing  brief  review  of  volcanic  eruptions  will,  in 
a  limited  way,  suffice  to  show  the  reader  the  immediate  physi- 
cal importance  of  these  accidents,  and  the  extent  to  which 
they  may  enter  into  the  conditions  of  human  life.  They  will 
not,  however,  give  him  any  measure  of  the  range  and  con- 
stancy of  this  volcanic  action,  or  the  part  it  plays  in  the 
machinery  of  the  earth's  crust.  To  gain  some  notion  of  this 
he  must  imagine  many  thousands  of  these  vents  scattered 
over  the  sea-floor  or  alonaf  the  shores  of  the  continents,  all 
of  which  have  been  active  in  recent  geological  times.  He 
must,  furthermore,  conceive  that  at  every  stage  in  the  earth's 
history  there  have  been  similar,  perhaps  equally  numerous, 
volcanoes  at  work.  It  is  doubtful  if  since  the  beginning  of 
the  geological  record  there  has  been  a  day  during  which  some 
crater,  great  or  small,  has  not  been  hurling  its  gases  toward 
the  sky,  scattering  its  dust  over  the  fields  of  land  and  sea, 
and  destroying  with  its  attendant  earthquakes,  or  by  its  ema- 
nations, the  life  of  air  or  water.  Lying  as  they  do  along  the 
shores  or  in  the  fertile  islands  of  the  ocean,  these  vast  engines 
of  destruction  are  a  perpetual  menace  to  many  of  the  most 
fruitful  and  beautiful  parts  of  the  earth  ;  they  therefore  have 
an  element  of  human  as  well  as  scientific  interest,  leading  us 
to  investigate  the  nature  of  their  cause  and  their  relation  to 
the  mechanism  of  this  planet. 

In  seeking  to  explain  any  of  the  superficial  phenomena  of 
our  globe,  it  is  well  to  begin  the  inquiry  by  considering  the 
manner  in  which  they  are  distributed  over  the  surface.  When 
we  have  clearly  delineated  on  a  map  the  distribution  of  any 
important  phenomena  over  the  surface  of  the  earth, — when 
this  distribution  is  effected  not  only  for  the  present  period,  but 
for  different  ages  in  the  past, — the  features  which  we  seek  to 


78  ASPECTS  OF  THE  EARTH. 

explain  are  assembled  in  such  a  manner  that  the  mind  readily 
takes  hold  upon  them.  We  are  then  almost  sure  to  be  led  by 
the  order  of  the  facts  to  a  rational  explanation  of  their  origin. 
All  the  more  important  discoveries  of  geology  have  evidently 
been  made,  however  clumsily,  by  this  method  of  inquiry. 
It  is  therefore  well  to  accept  it  as  a  principle  and  to  adopt 
the  method  in  all  fields  of  research  where  it  can  be  applied. 
In  this  way  we  are  most  likely  to  come  upon  a  clew  to  the 
origin  of  any  unexplained  feature  of  the  facts. 

A    glance  at    the  geographical  position  of  volcanoes  suf- 
fices to  show  us  that  they  are  very  peculiarly  grouped  in  and 
about  the  great  water-areas.      Probably  all  of  the  active  vents 
in  the  earth's  surface  lie  on  the  floor  of  the  oceans  or  greater 
seas,  or  within  a  few  score  miles  of  their  shores.     We  may, 
indeed,   say  that  active  volcanoes  normally  occupy  the  floor 
of    the    seas    as    their    proper    field,    and    that    this    volcanic 
area  here  and  there  overlaps  the  shore  for  a  very  small  dis- 
tance.     Moreover,  among  the  extinct  volcanoes  which  lie  far 
inland,  we  can  often  observe  that  their  activities  ceased  soon 
after  the  elevation  of  the  continent  forced  the  sea-margin  far 
from   their  bases.      It  was  long  ago  perceived  that  these  facts 
indicated  a  necessary  connection  between  the  effects  brought 
about  by  large  masses  of  water  and  the  volcanic  explosions. 
At    first    it    was    suggested    that    the    sea-water    penetrated 
through  crevices  to  the  heated  interior  of  the  earth,  and  there, 
being  converted  into  steam,  was  expelled  through  the  volcanic 
vent  along  with  the   lava  from  a  central   molten   mass.      But 
it  was  directly  seen  that  the    facts  were  against   this  hypoth- 
esis ;  for  why  should  the  volcanic   emanations  not   return  to 
the  surface  by  the  same  crevice  which  gave  the  water  access, 
to  the  earth's  interior  ?     Why  should   the  lava  of  ^tna  and. 


VOLCANOES.  79 

Other  volcanoes  rise  against  its  own  enormous  pressure  to  the 
height  of  twelve  or  fifteen  thousand  feet  above  the  tube  by 
which  the  sea-water  penetrated  to  its  base  ? 

It  has  since  been  suggested  that  the  water  from  the  seas 
gains  access  to  the  central  heat  while  it  is  imprisoned  in  the 
fine    interstices    which    lie   between   the   grains  of  the  rocks, 


Lake  of  Lava  in  the  Sandwich  Islands,  showing   Deposit  of  very  Fluid  Lava. 

passages  which  are  too  small  to  permit  the  exit  of  the  gases. 
A  curious  experiment  at  first  appeared  to  make  this  notion 
plausible.  As  was  shown  by  the  distinguished  naturalist 
Daubree,  if  we  take  a  vessel  of  metal  and  fix  upon  its  top 
a  sheet  of  dense  sandstone,  so  that  the  chamber  is  air-tight, 
then  place  water  upon  the  top  of  the  sandstone,  and  finally 
apply  heat  to  the  base  of  the  metal  chamber,  the  water  will 


8o  ASPEC7S    OF  THE  EARTH. 

penetrate  through  the  interstices  of  the  stone  and  generate 
steam  in  the  enclosed  space,  producing  a  pressure  which  is 
much  greater  than  the  gravitation-force  which  impels  the 
water  to  descend  through  the  stone.  If  we  provide  an  ave- 
nue of  escape  for  this  steam  by  means  of  a  pipe  filled  with 
mercury,  we  shall  find  that  it  will  force  the  mercury  up  the 
tube,  much  as  the  volcanic  steam  pushes  up  the  lava  in  the 
crater.  It  is  evident  that  we  have  here  what  seems,  at  first 
sight,  like  a  promising  explanation  of  volcanic  action  :  we 
have  only  to  conceive  that  water  penetrates  through  the  inter- 
stices of  the  rock  on  the  sea-floor,  just  as  it  does  through 
the  slab  of  sandstone  in  the  experiment  ;  that  the  internal 
heat  is  represented  by  the  lamp,  and  the  volcanic  tubes  with 
their  contained  lava  by  the  pipe  containing  mercury,  to  have 
the  likeness  complete.  But  a  little  consideration  shows  that 
this  explanation  will  not  serve  us  at  all.  It  is  true  that  the 
rocks  beneath  the  sea-floor  contain  a  good  deal  of  water — 
all,  in  fact,  that  their  interstitial  spaces  will  hold — but  this  is 
equally  true  of  the  rocks  beneath  all  parts  of  the  continents. 
The  rain-water  of  any  country,  however  slight  in  amount,  is 
suf^cient  to  fill  the  crevices  of  the  rocks  to  repletion,  if, 
indeed,  they  were  not  so  filled  when  they  were  formed  on  the 
sea-floors.  We  know  this  from  mines  in  the  land,  as  well  as 
by  many  galleries  which  penetrate  below  the  sea-level  from 
shafts  near  the  shore.  We  are,  therefore,  driven  to  another 
hypothesis,  which  is  entirely  satisfactory.  It  was  long  ago 
suggested,  though  it  has  not  been  presented  in  a  perfectly 
clear  form  in  our  popular  treatises  on  the  subject.  This 
explanation  may  be  stated  in  a  few  words  : 

When  deposits  of  rocky  matter  are    laid  down  upon  the 
sea-floor,  they  contain  a  good  deal  of  water.      Such  deposits 


VOLCAXOES.  8 1 

are  never  entirely  compact  ;  there  are  numerous  little  spaces 
between  the  grains  of  sand  or  mud,  in  or  between  the  fossil 
shells  and  other  animal  remains,  which  form  in  most  places  a 
part  of  the  strata  as  they  are  made.  We  see  how  large  an 
element  water  is  in  such  beds  if  we  take  up  a  portion  of  the 
mud  from  the  bottom  of  any  pool.  It  is  probable  that,  on 
the  average,  this  enclosed  water  amounts,  at  the  time  when 
the  deposits  are  made,  to  as  much  as  from  five  to  fifteen  per 
cent,  of  the  mass.  At  first  this  imprisoned  water  is  at  the 
ordinary  temperature  of  the  sea-floor,  and  so  has  no  tendency 
to  break  out  of  its  cells  ;  but  in  the  course  of  the  geologic 
ages,  a  great  many  thousand  feet  of  strata  are  slowly  accumu- 
lated above  the  original  level,  all  charged  in  the  same  way 
with  a  portion  of  the  fluid  in  which  they  were  laid  down.  We 
have  now  only  to  see  a  means  whereby  this  rock-encased  water 
can  be  raised  to  a  high  temperature — say  to  the  heat  of  two 
or  three  thousand  degrees,  Fahrenheit — in  order  to  bring  it 
to  the  state  of  the  steam  which,  escaping  from  rents  of  the 
earth,  gives  rise  to  the  explosions  of  volcanoes 

This  means  of  heating  is  provided  by  the  continuance  of 
the  very  process  which  builds  the  water  into  rocks,  viz.,  by 
the  deposition  of  strata  and  in  the  following  manner :  Heat  is 
constantly  escaping  from  the  earth's  interior,  which,  though 
probably  solid,  is  extremely  hot  ;  the  temperature  of  the 
central  portion  is  very  likely  to  be  measured  by  tens  of  thou- 
sands of  degrees.  Whenever  we  penetrate  by  wells  or  mines 
into  the  earth,  we  find  a  constant  increase  of  temperature  as 
we  descend.  It  is  likely  that  beneath  the  sea-floor  this  rate 
of  increase  is  somewhere  near  one  degree  to  every  fifty  feet  of 
depth,  varying  with  the  ease  with  which  the  heat  finds  its  way 
out  through  the  different  kinds  of  rocks  it  encounters.      Any- 

6 


82  ASPECTS   OF   THE  EARTH. 

thing  like  tliis  rate  of  increase  would  give  us  a  temperature  of 
several  hundred  thousand  degrees  at  the  earth's  centre. 

It  may  well  be  the  case  that  the  internal  heat  does  not 
increase  with  the  same  rapidity  as  we  descend  toward  the  cen- 
tral regions,  but  for  a  score  or  two  of  miles  this  increase  most 
likely  continues  at  something  like  this  proportion.  It  is  thus 
easily  seen  that  the  heat  of  any  mass  of  buried  rock  depends  on 
the  thickness  of  the  matter  deposited  above  the  level,  for  it  is 
that  blanket  of  strata  holding  in  the  heat  which  causes  its  tem- 
perature to  be  above  that  of  the  earth's  surface.  In  the  case 
of  a  deposit  made  on  the  sea-floor  and  covered  by  a  blanket  of 
strata  ten  thousand  feet  thick,  the  outflowing  tide  of  heat  will 
be  restrained  in  its  escape,  and  the  temperature  of  the  buried 
matter  will  in  time  rise  to  about  two  hundred  degrees  above 
the  temperature  which  it  had  at  first,  or  to  near  the  heat  of 
boiling  water.  Another  ten  thousand  feet  of  strata  may  raise 
the  temperature  high  enough  to  produce  some  of  the  slightest 
volcanic  explosions — those  in  which  the  rocks  are  not  melted, 
but  simply  blown  away — -while  with  a  deposit  one  hundred 
thousand  feet  thick,  the  rocks  might  in  time  hold  in  enough 
of  the  outflowing  heat  to  produce  the  most  intense  volcanic 
activity,  where  the  expanding  gases  act  with  more  than  the 
violence  of  gunpowder. 

If  the  reader  has  any  difficulty  in  conceiving  the  effects  of 
overlaid  beds  in  bringing  about  a  high  temperature  in  strata, 
he  may  help  himself  by  a  homely  comparison.  Let  him  ini- 
agine  a  vessel  containing  hot  water  exposed  to  the  cold  and 
covered  with  f(;lt  or  other  non-conducting  material  ;  the  sur- 
face of  this  covering  will  have  a  certain  temperature.  If  now 
this  vessel  be  covered  with  another  thickness  of  felt,  the  tem- 
perature of  the   original   surface  will   rise,  and   a  certain  gain 


rOLCAXOES. 


83 


of  its  heat  will   be   made   by  each   additional  coating-  of  non- 
conductive  material. 

The  only  serious  question  is  as  to  the  thickness  of  the 
rocks  which  have  been  laid  clown  on  the  sea-floors.  Hardly 
any   eeoloeist  will   doubt  that  it  is  entirelv  within   bounds  to 


Border  of  Lava-stream  in  the  Sandwich  Islands,  showing  the   Form  Assumed   by  Partly  Cooled   Lava.      Note 

the  "  Roping"  in  the  Lava. 

assume  that  thickness  much  to  exceed  twenty  miles.  It  may 
well  have  attained  to  twice  or  thrice  that  depth  since  the  geo- 
loo-ic  ao-es  beean,  for  in  our  continents  we  see  that  the  aggre- 
gate  thickness  of  the  successive  beds  exposed  to  view,  despite 
the  great  erosion  to  which  the  lands  have  been  exposed, 
amounts  to  somewhere  near  one  hundred  thousand  feet  of 
strata.       It    must  not  be  imagined  that  the  deposits  on  the 


84  ASPECTS    OF  THE  EARTH. 

floors  of  the  sea  were  ever  laid  down  in  water  having  the 
depth  of  ten  miles  or  more.  The  truth  is,  that  the  sea-floors 
have  been  gradually  sinking  as  the  lands  have  grown  upward. 
The  lands  have  furnished,  from  their  shores  and  from  the 
rivers,  sediments  which  have  gone  to  make  the  strata  which 
the  sea  has  deposited,  and  the  ocean-floors  have  slowly  bent 
downward  as  they  received  these  accumulations  of  waste.  As 
we  shall  shortly  note,  a  very  important  part  of  the  materials 
contributed  to  the  sea-bottoms  comes  from  the  volcanic  ejec- 
tions themselves.  We  thus  see  that  in  the  water  imprisoned 
in  the  deposits  of  the  early  geologic  ages  and  brought  to  a 
high  temperature  by  the  blanketing  action  of  the  more 
recently  deposited  beds,  we  have  a  suflicient  cause  for  the 
great  generation  of  steam  at  high  temperatures,  and  this  is 
the  sole  essential  phenomenon  of  volcanic  eruptions.  We  see 
also  by  this  hypothesis  why  volcanoes  do  not  occur  at  points 
remote  from  the  sea,  and  why  they  cease  to  be  active  soon 
after  the  sea  leaves  their  neighborhood.  While  deposition  of 
strata  is  going  on  with  moderate  rapidity,  as  it  generally  is 
over  the  sea-floors,  the  heat  is  constantly  rising  in  strata  and 
the  tendency  of  the  imprisoned  water  to  pass  into  steam  con- 
tinually increasing.  On  the  land  areas,  however,  the  rocks 
are  constantly  becoming  cooler,  and  the  expansive  energy  of 
the  steam  which  causes  the  eruptions  becomes  proportion- 
ately less. 

Conceiving,  then,  the  rocks  at  a  depth  of  ten  or  twenty 
miles  below  the  surface  of  the  earth  to  be  filled  with  steam  at 
a  temperature  near  two  thousand  degrees,  Fahrenheit,  we  may 
readily  explain  a  part  of  the  phenomena  of  volcanic  action, 
viz.,  the  formation  of  the  gases  essential  to  their  explosions. 
It  remains  for  us,  however,  to  account  for  certain   facts  con- 


VOLCANOES.  85 

cerning  the  movement  of  these  gases  toward  the  chance  open- 
ings by  which  they  find  their  way  to  the  surface  of  the  earth. 
It  may  well  be  asked,  Why  do  these  imprisoned  vapors  not 
make  their  way  directly  upward  through  the  rocks,  passing 
through  the  interstices  which  contain  the  water  ?     The  reason 

o 

for  this  doubtless  is,  that  as  the  cooler  rocks  above  are  very 
close-knit,  they  offer  much  the  same  obstacle  to  the  migrations 
of  the  steam  as  is  afforded  by  the  iron  walls  of  a  boiler.  The 
only  way  in  which  the  imprisoned  gas  can  escape  is  by  a  lat- 
eral motion  in  the  level  of  heated  and  softened  rocks  toward 
any  point  where  a  break  offers  them  passage  to  the  surface. 
Such  breaks,  extending  very  deeply  down  into  the  rocks,  are 
extremely  common. 

Let  us  imagine  such  a  break  or  fault  to  be  formed,  leading 
down  to  the  depths  of  imprisoned  water  where  the  rocks  have 
a  temperature  of  more  than  two  thousand  degrees,  Fahren- 
heit. At  once  the  water  near  the  opening  will  make  haste  to 
avail  itself  of  the  chance  of  escape.  As  it  is  contained  in 
every  part  of  the  imprisoning  rock  which  is  softened  by  heat, 
the  water  in  passing  to  the  point  of  escape  will  drive  the 
rock  before  it,  much  as  the  baker's  dough  is  moved  by  the 
imprisoned  gases  of  fermentation.  As  it  comes  to  the  surface 
the  steam  may,  to  a  great  extent,  escape  in  advance  of  the 
liquid  rock,  blowing  some  portion  of  it  to  bits  as  it  rushes 
into  the  air  ;  or  the  whole  of  the  softened  rock  may  be  blown 
into  dust,  as  in  the  greater  eruptions  we  have  before  noted. 
This  discharge  will  terminate  when  the  energy  of  the  outrush 
of  the  steam  is  so  far  diminished  that  the  column  of  lava  in 
the  volcanic  pipes  can  by  its  pressure  retain  the  vapor.  Then 
there  will  be  a  pause  of  some  duration. 

A  familiar  instance  of  the  process  by  which  a  mass  of  lava 


86  ASPECTS   OF  THE  EARTH. 

is  converted  into  pumice  maybe  found  in  the  slag  of  our  iron- 
furnaces.  This  slag,  as  is  well  known,  is  molten  rock,  and  in 
its  creneral  character  resembles  the  lava  poured  forth  from  vol- 
canic  vents.  Like  the  lava  of  the  volcano,  the  slag  of  the 
blast  furnace  is  under  a  considerable  pressure  when  it  escapes 
from  the  vent.  In  the  furnace  there  may  be  fifty  or  sixty  feet 
of  ore  and  fuel  above  the  level  whence  the  molten  iron  and 
slag  are  drawn.  In  the  volcano  there  may  be  a  thousand 
times  or  more  of  rock  material  above  the  lava  when  it  starts 
upward  through  the  throat  of  the  volcano.  When  the  slag  dis- 
charges, the  imprisoned  vapors,  hitherto  strongly  compressed, 
are  free  to  expand,  and  in  many  cases  they  blow  up  the  molten 
rock  which  contained  the  metal  into  a  spongy  mass,  produc- 
ing a  pumice  which  is  often  difihcult  to  distinguish  from  that 
formed  by  volcanoes. 

After  a  time  the  steam  from  regions  horizontally  remote 
from  the  point  of  escape  will  creep  in  toward  the  vent,  accumu- 
late pressure  there,  and  so  gradually  again  reproduce  the  con- 
ditions of  another  explosion.  As  this  imprisoned  steam  works 
toward  the  point  of  escape,  it  may  drive  before  it  the  rock  in 
which  it  is  contained,  and  so  furnish  a  continued  supply  of 
melted  material  for  the  discharge  of  ashes  and  lava ;  or  it  may 
pass  through  the  interstices  of  the  beds  without  forcing  the 
softened  rock  to  accompany  it.  We  have  many  evidences  of 
such  a  horizontal  movement  of  gases  alone,  or  of  rock  and 
gases  combined,  from  our  experience  in  mines  and  other  sub- 
terranean explorations.  When  in  a  deep  coal  mine  we  have 
horizontal  galleries  cut  in  beds  of  clay,  with  hard  rocks  above, 
we  often  find  that  the  clay  creeps  upward  from  the  bottom  and 
inward  from  the  sides  until  it  fills  the  cavity.  When  cut  out 
it  continues  the  movement,  putting  the  miners  to  much  trouble 


VOLCANOES. 


^7 


in  order  to  keep  the  way  open.  This  shows  us  how,  under 
the  inconsiderable  pressure  of  a  relatively  slight  weight  of 
overlying  beds,  rocks  which  seem  tolerably  hard  may  creep 
toward  a  point  of  relief.  Then,  again,  in  the  movement  of 
gases  contained  in  rocks,  we  have  evidence  that,  even  when 
urged  by  pressures  which  are  slight  compared  with  those  of 
the    volcano,  vaporous  matter  can  travel    for  a  considerable 


distance  through  ma- 
terials which  seem  to 
the  eye  to  be  compact. 
The  pressure  which 
impels    natural   gas  to- 

w:^rrl         the       bnrpd        well  Hypothetical   Sect.on   through    Rocks   near  a  Fault  on 

Wara        tne        OOrea       wen  wh,ch  a  Line  of  volcanoes  has  Formed, 

through      which      it      dis-        The  arrows  show  the  direction  of  the  movement  of 

gases  ;  their  length,  the  relative  energy  of  the  movement. 

charges   is  most   likely 

not  greater  than  a  thousand  pounds  to  the  square  inch.  This  is 
possibly  not  the  hundredth  part  of  that  which  impels  the  gases 
in  great  volcanic  explosions  ;  yet  as  a  well  will  sometimes  dis- 
charge ten  to  twenty  million  feet  of  gas  per  diem  for  years, 
it  is  evident  that  this  store  of  gas  must  be  derived  from  a  very 
wide  field.  It  is  probable  that  in  some  cases  it  may  journey 
for  miles  toward  the  outlet.  If  the  rocks  were  hot  it  would 
be  possible  for  the  imprisoned  gas  to  make  channels  of  escape 
by  blowing  the  rock  before  it.     We  can,  therefore,  well  im- 


88  ASPECTS   OF  THE  EARTH. 

agine,  in  the  case  of  the  volcanic  vapors,  that  owing-  to  their 
far  greater  pressure  and  to  the  softer  condition  of  the  rocks 
they  traverse,  they  may  migrate  for  hundreds  of  miles  to  the 
point  of  escape. 

It  seems  necessary  to  suppose  that  our  volcanoes  are  fed 
by  the  gases  and  lava  from  a  wide  field,  for  the  reason  that, 
notwithstanding  the  enormous  amount  of  materials  they  throw 
out,  the  ground  about  their  bases  rarely  if  ever  seems  to  be 
lowered.  For  Instance,  in  the  case  of  Vesuvius,  the  water  in 
the  form  of  steam,  the  lava  and  ashes  which  have  emanated 
from  it,  since  the  Christian  era,  have  amounted  probably  in 
all  to  more  than  five  cubic  miles,  yet  there  is  no  evidence  that 
the  cone  or  the  country  about  it  has  permanently  subsided  in 
that  time.  It  seems,  indeed,  here  and  there,  to  sway  up  and 
down  from  age  to  age,  but  the  average  height  above  the  sea 
remains  essentially  unchanged.  Unless  the  supply  of  the 
ejected  materials  comes  from  a  very  wide  subterranean  field, 
the  surface  of  the  reeion  should  show  a  decided  subsidence. 

The  evidence  of  this  sort  which  has  been  obtained  from 
the  study  of  yEtna  is  even  more  conclusive  than  that  which 
we  observe  in  the  case  of  Vesuvius,  ^tna  has  poured  out  a 
volume  of  discharged  matter  many  times  as  great  as  that 
w^hich  has  escaped  from  Vesuvius.  It  seems,  from  data  which 
cannot  be  considered  here,  that  since  the  beginning  of  its  his- 
tory JE\.x\2i  has  discharged  a  volume  of  vapor,  ash,  and  lava 
which,  when  buried  in  the  earth  below,  must  have  occupied  at 
least  one  thousand  cubic  miles  of  space,  and  yet  so  far  from 
the  foundations  of  the  mountain  having  sunk  down,  the  base 
of  the  cone  has  been  considerably  elevated  during  this  time 
when  material  was  coming  forth  to  the  earth's  surface.  The 
only   way   in   which  we  can    account    for    this  failure  of    the 


VOLCANOES. 


89 


ground  about  the  mountain  to  subside  is  by  the  supposition 
that  the  ejected  materials  in  the  main  are  derived  from  reg-ions 
remote  from  the  foundations  of  the  cone. 

The  foregoing  considerations  make  it  tolerably  clear  that 
volcanoes  are  fed  from  deposits  of  water  contained  in  ancient 
rocks  which  have  become  greatly  heated  through  the  blanket- 
ing efTect  of  the  strata  which  have  been  laid  down  upon  them. 


A.      Front  of  a  Lava-stream  Falling  in  Rivule»s  into  the  Sea,  Sandwich  Islands. 

The  gas  which  is  the  only  invariable  element  of  volcanic  erup- 
tions, is  steam  ;  moreover,  it  is  the  steam  of  sea-water,  as  is 
proved  by  analysis  of  the  ejections.  It  breaks  its  way  to  the 
surface  only  in  those  parts  of  the  earth  which  are  near  to 
where  the  deposition  of  strata  is  lifting  the  temperature  of 
water  contained  in  rocks  by  preventing,  in  part,  the  escape  of 
the  earth's  heat. 

In  the  extended  discussions  which  have  taken  place  among 


90  ASPECTS   OF  THE  EARTH. 

geologists  concerning  the  origin  of  volcanoes,  much  reference 
has  been  made  to  the  singular  pits,  apparently  of  volcanic 
origin,  which  are  extensively  scattered  over  the  surface  of  the 
moon.  In  fact,  the  side  of  our  satellite  which  is  turned  toward 
us,  and  probably  the  portion  which  is  turned  away  as  well,  is 
pitted  all  over  with  cavities  which,  in  a  general  way  at  least, 
seem  like  terrestrial  volcanoes.  In  all  cases  there  is  about 
the  crater-like  cavities  of  the  moon  walls  which  resemble  the 
cones  of  ordinary  volcanoes.  Although  there  is  a  general 
likeness  between  lunar  volcanoes  and  those  of  the  earth,  the 
differences  are  very  evident.  In  the  first  place  the  lunar 
craters  are,  unlike  the  terrestrial,  distributed  all  over  the  sur- 
face of  the  sphere  ;  they  are  not  arranged  with  distinct  order. 
In  size  they  vary  from  three  hundred  miles  or  more  in  diam- 
eter to  patches  which  are  not  more  than  a  few  hundred  feet 
across.  In  all  cases  the  crater  or  cup  is  very  wide  and  deep 
as  compared  with  the  surrounding  wall  of  the  cone. 

These  peculiarities  of  structure  and  of  distribution  make 
it  at  once  apparent  that  we  cannot  assume  an  exact  likeness 
between  tlie  lunar  and  terrestrial  volcanoes.  The  problem  is 
one  of  much  difficulty  and  cannot  adequately  be  discussed 
within  the  limits  of  this  chapter.  It  seems,  however,  most 
probable  that  the  lunar  volcanoes  were  formed  when  the  mass 
of  our  satellite  cooled  from  its  original  state  of  igneous  fluid- 
ity. The  pits  were  probably  produced  by  the  boiling  action 
during  which  the  gases  imprisoned  in  the  sphere  escaped  from 
the  surface.  For  some  reason  these  gases  have  not  remained 
as  an  atmosphere  about  tlie  moon.  It  is  a  reasonable  con- 
jecture that  they  have  been  absorbed  in  the  crevices  of  the 
very  porous  crust.  Owing  to  the  lack  of  any  permanent 
atmosphere,  the   moon   has   never  liad  any  water  upon  its  sur- 


VOLCANOES.  91 

face,  at  least  no  part  of  that  surface  now  exhibits  the  effect  of 
water  action.  It  is  j^robable  that  the  earth  was  at  one  time 
pitted  over  with  volcanoes  substantially  like  those  on  the 
moon,  which  were  due  to  the  boiling  which  took  place  as  the 
planet  cooled  ;  but  after  the  formation  of  these  aboriginal 
craters,  water  began  its  work  on  the  earth's  surface,  and  in 
time  it  entirely  effaced  the  original  volcanoes,  and  in  the  man- 
ner above  described  created  another  class  of  these  vents. 

A  careful  study  of  the  broad  low  cone  of  Mt.  yEtna  sup- 
plies us  with  an  excellent  illustration  which  serves  to  show 
the  general  method  in  which  volcanoes  of  the  type  of  those 
now  existino-  on  the  earth  are  formed.  On  the  flanks  of 
yEtna,  somewhat  remote  from  the  cone  of  eruption,  but  in 
the  wide  disc  over  which  the  lava  is  poured  and  the  ashes 
showered  in  great  abundance,  there  are  hundreds  of  small 
cones,  from  a  few  score  to  a  few  hundred  feet  in  height,  which 
are  the  results  in  most  cases  of  single  gas  eruptions  of  short 
duration.  The  history  of  these  little  craters  seems  to  be  as 
follows  :  The  loose-textured  materials  of  the  cone  contain  a 
p-ood  deal  of  water.  When  the  lava-streams  freeze  over,  the 
fluid  lava  below  the  surface  flows  out  and  leaves  caves  in  the 
manner  described  in  the  chapter  on  the  formation  of  caverns. 
In  time  these  subterranean  recesses  become  filled  with  water, 
which  percolates  from  the  surface.  When,  now,  the  heat  from 
the  underlying  lava  penetrates  to  these  masses  of  buried 
water,  there  to  turn  into  steam,  an  opening  is  broken  through 
the  overlying  material  and  a  brief  eruption  ensues.  Similar 
outbreaks  of  a  yet  more  easily  comprehensible  character  have 
been  observed  during  the  eruptions  of  Vesuvius.  As  has  been 
before  noted,  many  of  these  eruptions  have  invaded  the  vil- 
lacre  sites  on  the  flanks  of  the  mountain.      The  people  of  this 


92  ASPECTS    OF  THE  EARTH. 

district  secure  water  for  their  domestic  purposes  by  means  of 
cisterns,  the  porous  volcanic  deposits  affording-  no  wells  or 
running  streams.  When  the  current  of  lava  passes  over  one 
of  these  cisterns,  the  stone  coping  of  the  reservoir  commonly 
prevents  the  ingress  of  the  semi-fluid  lava  into  the  cavity  until 
a  considerable  thickness  of  material  has  gathered  above  it. 
When  finally  the  lava  comes  in  contact  with  the  water,  a 
quantity  of  steam  is  produced,  which  for  a  time  blows  up  a 
cone  of  eruption  about  the  point  where  the  vapor  escapes. 
When  the  supply  of  water  is  exhausted,  the  flow  of  lava 
destroys  tlie  little  cone. 

From  tiiese  theoretical  considerations  as  to  the  causes  of 
volcanoes  it  will,  perhaps,  be  a  relief  to  the  reader  to  turn 
to  the  question  of  their  place  in  the  economy  of  the  earth. 
Although  volcanoes  are  agents  of  great  destructive  violence, 
we  easily  see  that  they  render  an  immeasurable  service  to  the 
earth  by  returning  to  its  surface  a  great  store  of  materials 
which  are  necessary  to  the  functions  of  life  and  which  are  con- 
stantly being  buried  in  the  deeper  parts  of  the  crust,  and  so 
withdrawn  from  the  activities  characteristic  of  the  superficial 
part  of  the  globe.  Let  us  consider,  in  the  first  place,  the 
action  of  volcanoes  in  returning  buried  water  to  the  seas.  We 
have  not("d  the  fact  that  when  strata  are  deposited  on  the  sea- 
floor  they  contain  a  large  amount  of  water;  it  is  probably  safe 
to  assume  that  on  the  average  not  far  from  ten  per  cent,  of 
the  mass  consists  of  this  material.  As  the  average  depth  of 
the  oceans  is  not  far  from  fifteen  thousand  feet,  it  is  evident 
that  the  amount  of  water  thus  abstracted  by  the  deposition  of 
strata  from  the  earth's  surface,  in  the  course  of  the  geologic 
ages  since  the  ocean  came  uj)on  the  surface  of  the  earth,  has 


Illllll 


111 
ill 


VOLCANOES. 


93 


been  very  great.  If  the  thickness  of  the  part  of  the  crust 
which  has  been  laid  down  on  sea-floors  amounts  to  as  much 
as  one  hundred  and  fifty  thousand  feet,  the  oceans  might  have 
disappeared  in  their  own  deposits,  and  so  the  surface  of  the 
earth  would  have  had  a  limit  put  to  its  most  important  pro- 
cesses.      But  by  the  operations  of  the  volcano  a  large  part 


B.      The  Same  Lava-stream  Pouring  in  Full  Tide  into  the  Sea. 

of    the  imprisoned  water  is    in    time   restored  to  the  earth's 
surface,  and  so  reenters  on  its  beneficent  activities. 

With  the  steam  from  a  volcano  there  comes  forth  also  a 
considerable  amount  of  the  carbonic-acid  gas  which  must  be 
present  In  the  air,  else  vegetation  would  cease  to  be.  A  very 
great  amount  of  this  substance  is  each  year  taken  from  the 
atmosphere  and  buried  in  the  earth,  not  only  by  the  plants 
and  animals,  the  carbon  of  whose  remains  are  buried  in  strata. 


94  ASPECTS   OF  THE  EARTH. 

but  also  by  certain  processes  of  decay  of  rocks,  as  where  the 
felspar  of  granitic  materials  is  converted  into  kaolin.  About 
the  only  manner  in  which  this  carbon  can  find  its  way  back 
into  the  air  is  through  volcanic  action.  It  is  not  likely  that 
volcanic  activity  can  restore  enough  of  this  carbon  in  the  form 
of  carbonic-acid  gas  to  compensate  for  the  constant  and  rapid 
burial  of  the  substance  in  the  earth,  but  it  is  certainly  a 
means  whereby  a  good  deal  of  it  is  returned  to  the  atmos- 
phere. In  certain  cases  the  emanation  of  this  combined  oxy- 
gen and  carbon  from  volcanoes  is  in  such  volume  that  it 
is  extremely  destructive  to  life  ;  being  a  heavy  gas,  it  flows 
like  water  down  the  sides  of  the  cone,  carrying  with  it  death 
to  all  animals.  Such  destructive  effects  are  limited  to  the 
first  and  last  stages  of  an  eruption.  When  a  volcano  is 
reduced  to  its  last  stages  of  activity,  when  it  is  only  a 
smouldering  vent,  it  often  continues  to  pour  forth  this  gas 
long  after  it  has  ceased  to  produce  any  other  evidence  of  its 
connection  with  subterranean  processes.  A  good  case  of  this 
is  seen  in  the  Solfatara,  near  Naples,  where  a  small  crater, 
long  since  extinct  as  a  volcano,  throws  out  enough  carbonic 
acid  to  suffocate  a  dog,  to  the  diversion  of  hard-hearted  tour- 
ists and  the  profit  of  the  proprietors  of  the  brutal  show. 

The  solid  matter  thrown  out  by  volcanoes  is  the  most 
important  contribution  to  the  materials  which  the  sea  has  at 
its  disposal  for  the  nourishment  of  its  life  and  for  the  forma- 
tion of  strata.  The  quantity  of  the  pumiceous  and  finely 
pulverized  material  is,  as  we  have  seen,  enormous.  When 
it  falls  upon  the  sea  it  either  floats  for  a  time  or  at  once 
sinks  into  the  depths.  In  either  case  it  is,  to  a  great  extent, 
dissolved  in  the  ocean  waters,  and  so  contributes  to  the  store 
of  materials  which  may  be  appropriated  by  the  organic  life  of 


Rent   in   the   Earth  from    which   Sulphurous    Vapors   Attendant  on   an    Eruption    have    Escaped;     Partly   Closed   by 

Tropical   Vegetation. 


VOLCANOES.  95 

the  sea.  When  it  falls  on  the  land,  it  is  generally  so  incohe- 
rent that  it  is  easily  swept  away  by  the  rains,  and  so  comes 
quickly  into  the  ocean.  The  Importance  of  this  contribution 
to  marine  sediments  has  been  overlooked  by  geologists,  but 
it  is  easy  to  see  that  it  may  amount  in  mass  to  something  like 
as  much  as  the  earthy  matter  which  is  brought  to  the  sea  by 
the  rivers.  The  volcanoes  of  the  Java  district  alone  have 
within  a  century  thrown  out  a  mass  of  this  fragmentary  rock 
amounting  probably  to  not  less  than  one  hundred  cubic  miles, 
and  perhaps  to  twice  this  quantity.  Now,  the  Mississippi 
River  carries  out  In  the  form  of  dissolved  matter,  mud,  and 
sand  about  one  cubic  mile  In  tVv-enty  years,  or  five  cubic  miles 
in  a  century  ;  thus  these  volcanoes  of  the  Java  district  have 
brought  up  from  the  depths  of  the  earth  and  contributed  to 
the  sea  many  times  as  much  detritus  as  has  been  conveyed 
to  the  ocean  by  the  greatest  river  of  North  America.  Allow- 
ing for  the  greater  porosity  of  the  volcanic  ejections,  it  still 
seems  not  unlikely  that  the  mass  of  rocky  matter  from  a  half 
dozen  great  volcanoes  of  the  East  Indian  Archipelago,  in  the 
period  of  a  little  more  than  a  century  from  1772  to  1883, 
far  exceeded  that  brought  Into  the  oceans  by  all  the  rivers 
of  North  America  In  the  same  period.  Although  the  volca- 
noes of  this  district  are  by  far  the  most  powerful  which  are 
known,  we  still  cannot  fairly  reckon  that  their  ejections  rep- 
resent anywhere  near  the  half  of  the  total  quantity  which 
came  to  the  earth's  surface  from  such  vents  during  the  above- 
named  period  of  one  hundred  and  eleven  years.  For  during 
this  time  some  scores  of  great  craters  were  in  eruption,  in- 
cluding Skaptar  in  Iceland,  Vesuvius,  ^tna,  various  volca- 
noes in  South  America  and  elsewhere.  It  seems,  therefore, 
not  unlikely  that  the  solid  materials  contributed  by  volcanoes 


90 


ASPECTS    OF  THE  EARTH. 


to  the  sea-floor  may,    on  the  average,  amount  to   more  than 
that  taken  by  the  rivers  from  the  land. 

A  short  time  ago  I  had  an  excellent  opportunity  for  per- 
ceiving the  amount  of  the  pumiceous  matter  which  is  carried 
about  the  ocean,  by  marine  currents.      On  a  journey  up  the 


W  dt  Lli  .  1    ;,t;.-,im  at  Hjintot  tgress,  showing  very   f- lu id  Condition,  with  Escaping  Steam,  Sandwicn 

Islands. 

east  coast  of  Florida,  from  Key  West  to  the  northern  part 
of  Indian  River,  I  observed  that  the  sea-shore  was  abundantly 
strewn  with  bits  of  pumice.  On  a  close  examination  I  found 
at  least  one  bit  of  pumice  to  f*ach  one  hundred  feet  on  the 
length  of  the  shore,  and  on  examining  the  santl  perceived 
that  a  large  part  of  the  material  was  composed  of  porous 
volcanic  matter  which  had  been  ground  by  the  action  of  the 
waves  into  small  bits.  It  is  evident  that  the  Gulf  Stream 
^'s  charofed  with  this  volcanic  material,   and   the  considerable 


VOLCANOES.  97 

variety  in  the  mineral  character  of  the  specimens  appears  to 
indicate  that  they  have  journeyed  from  many  different  parts 
of  the  earth.  It  is  probable  that  all  our  sandy  sea-shores 
contain  bits  of  rock  which  have  been  floated  in  the  form 
of  pumice  from  the  remoter  parts  of  the  oceans. 

Among  these  solid  substances  which  are  ejected  by  volca- 
noes we  find  some  of  the  most  indispensable  elements  of 
organic  life,  including  phosphorus,  soda,  potash,  and  other 
materials.  The  value  of  these  materials  to  vegetation  may  be 
judged  by  the  fertility  which  so  often  characterizes  the  regions 
in  the  immediate  vicinity  of  volcanic  cones  which  cast  forth 
large  amounts  of  ash.  If  the  rainfall  be  sufficient  this  ash 
quickly  decomposes  into  a  fertile  soil,  which  tempts  the  hus- 
bandman to  replant  the  fields  as  fast  as  they  are  ravaged  by 
the  explosions.  Were  it  not  for  the  constant  return  of  these 
rarer  and  precious  materials  to  the  superficial  part  of  the 
earth  by  means  of  volcanic  action,  it  is  likely  that  the  earth's 
surface  would  want  many  of  the  substances  most  necessary 
for  organic  life. 

We  thus  see  that  volcanoes  play  a  ver)  important  part  in 
the  physical  history  of  our  planet.  The  action  is,  in  a  large 
degree,  restorative.  They  help  to  maintain  the  earth's  sur- 
face in  a  condition  in  which  it  may  nurture  life.  We  note 
also  that  this  internal  heat  of  the  earth,  acting  through  vol- 
canoes, serves  to  counteract  certain  injurious  effects  arising 
from  the  operation  of  the  solar  forces.  The  heat  of  the  sun 
operating  in  the  rivers  and  the  waves  wears  away  the  materi- 
als of  the  land,  buries  them  in  the  strata  of  the  sea-floor  along 
with  a  part  of  the  water  of  the  seas.  The  internal  heat  expels 
the  most  volatile  and  the  most  life-giving  portions  of  these 
substances,  affording  them  a  chance  to  take  their  places  once 
again  within  the  activities  of  the  surface. 


CAVERNS   AND   CAVERN    LIFE. 


Effect  of  Caverns  on  Imagination. — Classification  of  Caverns. — Limestone  Caves:  Method 
of  Formation. — Caverns  of  Kentucky. —Sink  Holes,  .Shafts,  Domes,  and  Calleries. — 
Formation  of  Stalactites. — Natural  Bridges. — Air  of  Caverns  :  Effects  on  Decay  ;  on 
Health. — Relation  of  Primeval  Man  to  Caverns  ;  Dwellings;  Burial-Places. — Remains  of 
Animals. — Living  Animals  of  Caverns  ;  Bearing  of  Evidence  on  Darwinian  Tlieory. — 
Geographical  Distribution  of  Limestone  Caverns. — Hot  Spring  Caves  ;  Mineral  Depos- 
its in  such  Caverns.  —  Fault  Caverns. — Wave  Caverns  :  Blue  Grotto  ;  Staffa  Cave. —  Rock 
House  Caves. — Lava  and  other  Volcanic  Caverns. — Symmes's  Hole. 

The  surface  phenomena  of  the  earth,  the  scenes  which 
have  an  every-day  famiharity,  soon  become  to  ordinary 
observers  commonplace.  The  sailor  finds  the  ocean  tire- 
some, and  the  dweller  of  the  Alps  sees  little  to  awaken 
pleasurable  emotions  in  the  peaks  and  glaciers  which  from 
year  to  year  meet  his  eyes.  All  of  us  are  familiar  with  the 
glory  of  the  starlit  sky,  and  know  that  these  points  of  light 
are  the  spheres  of  planets  and  suns  scattered  through  fathom- 
less space  ;  and  yet  this  spectacle,  which  would  overwhelm 
the  soul  were  it  disclosed  for  a  single  hour  in  a  lifetime, 
arrests  but  a  momentary  interest  or,  oftener,  passes  unnoticed 
at  all.  It  is  the  unseen  which  most  attracts  us.  Thert-fore, 
in  all  times  men  have  speculated  as  to  the  contents  of  the 
nether  earth.  Its  crevices  and  caverns  afford  in  their  dark 
recesses  a  world  wliich  tlic  imagination  can  people  at  its  will. 
Even  if  they  excite  only  a  vague  wonder  mingled  with  terror, 
these  subterranean   spacers  are  still  fascinating  to  the  explorer 


CA  vjlRNS  axd  ca  VjErn  life.  99 

weary  of  the  well-known  or,  rather,  familiar  objects  of  the  sun- 
lit world. 

The  class  of  underground  openings  known  as  caverns  have, 
in  all  countries  and  at  all  times,  been  especially  captivating  to 
the  lovers  of  the  marvellous  ;  their  strange  architecture,  beau- 
tiful ornamentation,  and  peculiar  inhabitants  have  combined 
to  make  them  attractive.  To  men  of  science  they  have 
recently  become  extremely  interesting,  because  they  throw 
licrht  on  the  early  conditions  of  savage  man,  and  in  other 
ways  make  some  startling  contributions  to  the  facts  which 
bear  on  the  so-called  Darwinian  theory. 

The  open  spaces  of  the  underground  may,  for  conven- 
ience, be  divided  into  several  distinct  classes :  First,  we  have 
the  caverns,  or  the  channels  excavated  in  limestone  rocks  by 
streams  which  find  their  way  beneath  the  surface.  Next,  the 
channels  and  chambers  hollowed  out  by  the  waters  of  hot 
springs  on  their  way  from  the  depths  of  the  earth  to  the  sur- 
face. Third,  come  the  sea-caves,  formed  where  the  battering 
surges  have  worn  a  way  into  the  shore-cliffs  along  the  line 
of  some  softer  part  of  the  rocks  or  of  an  incipient  fissure. 
Fourth,  the  cavities  curiously  formed  where  a  lava-stream  has 
frozen  or  solidified  on  the  surface,  while  the  liquid  rock  below 
has  flowed  on  or  sunk  back  into  the  depths,  leaving  the  arch 
standing,  until  the  matter  which  originally  supported  it  has 
disappeared.  Lastly,  we  have  the  rifts  formed  in  the  rocks 
which  have  been  rent  by  the  mountain-building  forces,  where 
the  walls  on  either  side  of  the  break — or,  as  it  is  termed  by 
miners,  the  fault — have  been  pulled  apart  from  each  other, 
leaving  a  very  deep  and  long,  but  relatively  narrow,  fissure. 
In  one  or  another  of  these  groups  we  may  place  all  the  known 
cavities  which  occur  beneath  the  earth's  surface.      The  variety 


lOO  ASPECTS    OF  THE  EARTH. 

of  these  subterranean  chambers  is  so  hmited  that  we  shall  be 
able  within  the  compass  of  this  chapter  to  see  something  of 
the  history  and  character  of  them  all. 

Owing  to  their  wide  distribution,  great  variety,  and  vast 
extent,  the  limestone  caverns  are  the  most  interesting  of  these 
groups  of  caves.  They  occur  in  all  those  parts  of  the  earth's 
surface  where  thick-bedded  and  pure  limestones  lie  with  their 
layers  somewhere  near  horizontal,  and  where,  at  the  same 
time,  the  main  streams  have  cut  deep  channels  in  the  surface 
of  the  country.  It  is  also  essential  that  the  region  should  be 
forest-clad  ;  or,  even  if  now  deforested,  that  it  should  have 
been  covered  by  woods  at  the  time  when  the  excavation  of  the 
caverns  was  going  on.  With  these  conditions  the  formation 
of  caverns  is  necessarily  brought  about.  The  rain-water  fall- 
ing on  the  surface  of  the  decaying  vegetation  has,  when  it 
arrives  on  the  earth,  but  little  power  of  dissolving  rocks  of 
any  kind  ;  but  on  passing  through  this  bed  of  oxidizing 
carbon  it  takes  up  a  large  amount  of  the  gaseous  material, 
composed  of  one  atom  of  carbon  and  two  of  oxygen,  known 
commonly  as  carbonic-acid  gas.  This  absorbed  gas  gives  the 
water  a  singular  capacity  for  taking  into  solution  a  large 
amount  of  lime,  iron,  and  many  other  substances  which  are 
found  in  rocks. 

Descending  througli  the  soil,  this  solvent  compound  of 
water  and  carbonic-acid  gas  finds  its  way  into  the  narrow 
crevices  or  joint-phuK^s  which  exist  in  all  rocks.  It  (piickly 
widens  these  channels  until  they  are  so  spacious  tliat  the 
brooks  desert  the  surface  and  become  underground  streams, 
which  often  course  for  miles  in  the  hidden  channels.  At  first, 
while  the  crevices  are  narrow,  the  excavation  is  altogether 
done  bv  the  dissolving  action   of  the  water  ;  but  when   it  has 


CArA/^xs  Axn  caverx  life.  ioi 

thus  excavated  a  channel  sufficiently  large  to  permit  a  stream 
to  How  freely  through  it,  the  speed  of  the  current  through 
the  new-found  way  abrades  the  rocks  by  its  mechanical  power, 
at  the  same  time  exercising  its  solvent  action.  To  see  the 
nature  and  extent  of  this  work,  we  should  go  to  some  district 
of  extensive  limestone  caverns  and  examine  the  action  of  the 


C.v.,    Hill,   with    Smk-lioles,    L..ray,   Va. 

water,  from  the  time  when  it  falls  on  the  surface,  along  the 
course  of  its  underground  journey,  to  the  point  where  it 
emerges  beneath  the  cavern's  arch  into  the  main  river  of  the 
country. 

Probably  the  best  region  in  the  world  for  the  study  of  this 
interesting  geological  work  is  the  caverned  district  about  the 
head-waters  of  the  Green  River  in  Kentucky.  In  that  region 
the  limestones  of  the  Subcarboniferous  group  of  rocks  attain 
a  depth  of  several  hundred  feet,  and   are  very  thick-bedded, 


I02  ASPECTS   OF  THE  EARTH. 

the  separate  layers  or  beds  being  often  twenty  or  thirty  feet 
thick.  The  pure  nature  of  this  Hmestone,  and  the  absence  of 
divisional  planes,  such  as  the  thin  beds  of  clay  which  com- 
monly divide  such  deposits,  are,  as  we  shall  see,  peculiarly 
favorable  to  the  formation  of  wide  and  lofty  caverns.  This 
thickness  of  the  beds  is  due  to  a  cause  which  it  is  interesting 
to  note  ;  for  the  reason  that  it  shows  how  dependent  the 
shape  of  our  earth  is  upon  the  nature  of  the  creatures  that 
build  with  their  remains  the  rocks  which  form  on  the  sea- 
floor.  The  greater  part  of  the  limy  matter  in  limestones  is 
formed  of  the  remains  of  animals  which  lay  prone  upon  the 
sea-floor.  When  any  great  disturbance,  such  as  earthquake- 
shocks,  agitated  the  water  on  that  floor,  the  slimy  mud  which 
was  swept  about  destroyed  over  wide  areas  this  population  of 
the  sea-bottom.  Until  these  creatures  re-established  them- 
selves, the  sediments  which  were  formed  would  not  contain 
much  lime,  but  would  consist  of  clayey  or  sandy  matter  alone. 
If  this  process  were  often  repeated,  the  resulting  limestone 
would  be  so  frequently  interrupted  by  insoluble  layers  of 
other  materials  that  only  shallow  and  unimportant  caverns 
would  be  developed  in  them. 

There  are  two  ways  in  which  these  massive  limestones  can 
be  formed  in  the  deeper  seas  :  As  in  the  central  part  of  the 
North  Atlantic,  where  minute  limestone-encased  creatures 
float  in  the  water  while  they  live,  and  at  their  death  give  their 
skeletons  to  the  sediments  of  the  sea-floor  ;  in  wliich  way  mas- 
sive limestones,  such  as  the  chalk  dejiosits  of  P^ngland,  have 
been  produced.  Another  method  in  which  such  deposits 
are  made — the;  way,  indeed,  in  wliicli  these  subcarboniferous 
limestones  of  the  Mississippi  valley  were  formed — is  by  the 
following  ])n)C('ss  :   C('rtain   of  the   tenants  of  the   sea-floor — 


CA  J'i:7i.\S  AXD   CA  VERX  LIFE. 


103 


the  corals,  and  especially  the  sea-lilies — have  stems  which  lift 
the  mouths  of  the  creatures  above  the  level  of  the  frequently 
stirred  mud  ;  thus  they  survive  the  catastrophes  which  bring 
death  to  the  sensitive  forms  whose  bodies  become  buried  in 
the  running  slime.  The  greater  part  of  the  animals  which 
contributed  their  remains  to  these  massive  beds  of  lime  were 
of  these  stemmed  groups,  and  this  slight  peculiarity  has  given 


"*"*"^^J<<^ 


Smk-holes,  Edmonson  County,  Ky. 

rise  to  the  features  which  so  mark  this  country  over  a  region 
of,  at  least,  ten  thousand  square  miles  in  area. 

As  soon  as  the  observer  comes  upon  this  caverned  district 
of  Kentucky  he  remarks  that  he  has  passed  from  the  region 
where  running  brooks  abound,  and  is  in  a  country  where  there 
are  neither  streams  nor  the  distinct  hills  and  valleys  which 
he  is  accustomed  to  see  in  other  lands.  The  surface  of  this 
area  is  cast  into  a  series  of  shallow,  circular  pits,  varying  in 
diameter  from  a  few  score  feet  to  half  a  mile  or  more.  So 
crowded  together  are  these  pits  that  almost  the  entire  surface 


I04 


ASPECTS    OF  THE  EARTH. 


lies  in  some  one  of  these  depressions.  In  the  bottom  of  each 
of  the  pits  there  is  normally  a  vertical  shaft,  or  a  series  of 
crevices,  down  which,  in  time  of  rain,  the  water  flows  from 
the  drainage-slope  of  the  pit,  or  "  sink-hole  "  as  it  is  called  in 
local  phrase.     Generally  these  conduits  have  been  closed,  by 


'"^'^iwiil^' 


Sink-hole,  Edmonson  County,  Ky.     (The  shaft  leading  down  to  the  cavern   has  been  artificially  closed.) 

accident  or  design,  in  whicli  case  a  little  pool  of  circular  out- 
line occupies  the  centre  of  the  depression.  Occasionally,  in 
place  of  the  sieve-like  openings  which  usually  give  the  rain- 
water passage  to  the  depths  of  the  earth,  the  opening  is  large 
and  circular,  resembling  the  entrance  to  a  well.  Such  open- 
ings  were    once    common    in    tliis    country  ;     but    the    cattle, 


CA  J^£/^XS  AXD   CA  VERN  LIFE.  105 

tempted  by  the  rich  herbage  which  commonly  grew  about  the 
damp  borders  of  the  pit,  were  often  entrapped  in  the  opening, 
so  the  greater  number  of  them  have  been  artificially  closed. 
Now  and  then  in  the  remaining  forest-areas  we  may  find 
these  shafts  still  remaining  open,  offering  the  way  for  daring 
explorations,  which  we  are  about  to  invite  the  reader  to  follow 
in  his  imagination. 

The  ordinary  visitor  to  this  region  of  caverns  enters  the 
few  show-caves  in  the  convenient  way  afforded  by  some  break 
of  their  roofs,  or  by  the  old  places  of  exit  of  the  caverning 
streams.  In  actual  practice  we  commend  this  conservative 
custom  ;  but  as  our  imaginary  journey  demands  only  ideal 
risks,  we  may  now  proceed  to  follow  the  history  of  the  pro- 
cess of  cavern-making,  from  the  place  where  it  begins  to  the 
point  where  the  waters  conclude  their  underground  work  and 
enter  the  open  streams."' 


*  Making  a  simple,  strong  frame  over  the  opening,  to  hold  a  hoisting-block,  and 
passing  a  strong  rope,  some  hundred  feet  in  length,  through  this  block,  the  explorer 
will  have  the  means  of  descending  to  the  nether  world.  It  will  be  well  for  him  to 
take  the  precaution  of  fastening  the  rope  around  his  left  ankle,  with  a  well-arranged 
slip-knot,  and  then  place  the  same  foot  in  a  simple  stirrup-loop  of  the  rope.  Thus, 
in  case  he  should  by  any  chance  lose  hold  of  the  rope,  he  cannot  fall  into  the 
depths.  A  signal-cord  should  also  be  provided,  by  which  the  explorer  can  send 
the  simple  commands  of  lower,  stop,  hoist  y  for  the  depth  and  width  ot  the  vault 
into  which  he  descends  may  be  so  great  that  his  voice  will  be  lost  in  the  space 
or  confused  by  reverberation.  This  cord  should  be  fastened  to  the  waist,  and 
should  be  led  to  one  side  of  the  opening,  so  that  it  may  not  become  wound  round 
the  main  rope.  In  practice  it  requires  four  trusty  helpers  to  manage  this  explora- 
tion—three to  control  the  rope,  and  one  for  the  signal-cord.  In  fact,  it  requires 
five  people  who  are  not  apt  to  become  nervous,  for  the  explorer  himself  should  be 
a  calm-minded  person. 

The  explorer  should  take  with  him  an  oil-lantern,  holding  six  hours'  supply  ;  at 
least  two  candles,  well  fastened  in  his  pockets  ;  and  two  water-proof  match-boxes, 
and  some  bits  of  magnesium-wire  or  bengal-lights   for  illumination.     A  stout  staff 


lo6  ASPECTS   OF  THE  EARTH. 

With  proper  precautions,  the  most  important  of  which  are 
indicated  in  the  foot-note,  the  adventurous  person  may  de- 
scend these  pits  with  no  more  risk  than  he  encounters  in 
Alpine  mountain-work.  In  this  country,  where  untrodden 
heights  are  not  open  to  us,  it  may  be  worth  while  for  the  lover 
of  adventure  to  try  these  cavernous  depths.  The  present 
writer,  who  has  tried  both  kinds  of  exploration,  is  inclined 
to  consider  the  cavern-work  as,  perhaps,  the  more  fascinating- 
of  the  two.  Certainly,  the  explorer  more  quickly  finds  his 
way  into  the  realm  of  the  unknown  than  in  mountain-climb- 
ing, and  is  less  often  met  by  the  discouraging  evidence  that, 
after  all,  the  ground  is  not  untrodden. 

The  first  thing  we  note  on  entering  the  throat  of  the 
chasm  is  that,  if  it  be  warm  weather,  there  is  a  decided  cur- 
rent of  air  setting  down  into  the  space  below  ;  if  it  be  cold, 
there  is  an  ascending  current  of  warm  air  from  the  shaft, 
which  condenses  into  mist  as  it  escapes  from  the  opening. 
The  meanine  of  these  currents  we  shall  see  when  we  come  to 
consider  the  movements  of  the  air  in  caves. 

Descending  a  few  feet  into  the  chasm,  we  note  that  the 
shaft  rapidly  widens  on  every  side,  so  that  in  most  cases  we 
quickly  lose  sight  of  the  bordering  walls  ;  the  structure  of  the 
shaft  is,  indeed,  that  of  a  rude  dome,  of  which  the  hard  layer 
at  the  top  forms  the  capstone.  After  going  down  a  little 
distance,  the  width  becomes  so  great  that  the  scant  light  of 
a  single  lantern  may  not  disclose  the  sides  of  the  arch.  At 
a  depth  of  a  few  more  feet,  we  find  that  the  pit  again  con- 
tracts,  a    great  shelf  extending   from    the    sides    to    near  the 

with  a  thong,  by  which  to  hang  it  to  the  waist,  will  be  useful.  Care  should  be  taken 
that  the  rope  is  several  times  as  stron.i;  as  is  required,  and  that  it  has  no  tendency  to 
spin  round  when  a  weight  is  put  upon  it. 


CA  J^£I^AS  AND   CA  VERN  LIFE. 


107 


centre,  through  which  there  is  a  passage  rather  wider  than 
that  at  the  orifice.  Landing  on  this  shelf,  we  find  it  to  be 
a  tolerably  level  floor,  from  which  spring  the  walls  of  the 
upper  dome  ;  from  one  or  more  sides  of  it  extend  galleries, 
whose  floors  lie  on  this  harder  layer — their  arches  excavated 


Stalactites,  Luray   Cavern.     (Engraved  fronn  a  photograph   by   C.  H.  Janes.) 

in  the  softer  overlying  rock.  We  see  at  a  glance  that  these 
channels  were  once  the  paths  of  streams,  though  they  have 
not  for  ages  been  occupied  by  their  waters.  As  we  follow 
down  the  wandering  gallery,  we  find  that  it  is  joined  by  many 
similar  passages,  the  whole  forming  a  labyrinth  in  which  the 
unwary  explorer  may  easily  become   confounded.      Each  of 


lo8  ASPECTS    OF  THE  EARTH. 

these  passages  terminates  in  a  vertical  shaft,  or  rude  dome, 
essentially  like  that  by  which  we  gained  access  to  the  cavern, 
but  generally  communicating  with  the  external  air  by  passages 
so  narrow  and  tortuous  that  they  do  not  admit  the  light.  We 
can  see  that  as  this  main  channel  is  joined  by  the  side  pas- 
sages it  constantly  increases  in  size,  until,  perhaps,  it  attains 
majestic  dimensions.  We  may  travel  through  it  for  miles, 
until  we  are  suddenly  arrested  by  some  one  of  several  classes 
of  obstacles :  A  great  fall  of  stones  from  the  roof  may  close 
the  way  ;  or  through  the  hard  layer  which  constitutes  the  floor 
the  water  may  have  found  and  enlarged  a  downward  passage, 
creating  a  dome  like  that  which  we  descended  ;  or,  more  fre- 
quently, an  assemblage  of  crowded  stalactitic  pendants  and 
columns  close  the  once  open  space  as  with  a  wall  of  resplend- 
ent crystals.  Returning  to  the  main  dome,  we  may  continue 
the  descent  toward  the  lower  level  of  the  cavern.  In  the 
depth  below  the  first  level  of  galleries  we  find  several  others, 
each  having  the  same  general  character,  and  all,  in  turn, 
deserted  by  streams,  each  with  the  infinite  variety  of  detail 
given  by  the  eddying  current  of  the  vanished  streams  and 
the  trickling  waters  which  bring  in  the  stalactitic  materials. 
Finally,  we  come  to  the  floor  of  the  cave,  and  commonly  land 
in  a  considerable  pool  of  water,  partly  filled  with  angular  frag- 
ments of  flint.  In  times  of  heavy  rain,  when  the  waters  pour 
down  this  great  shaft,  these  fragments  of  hard  stone  are  set 
into  tumultuous  motion,  and  for  a  time  rapidly  work  through 
the  hard  floors  which  the  shaft  encounters  in  its  downward 
progress.  There  is,  however,  a  limit  to  their  wearing  action  ; 
for  when  this  pool  attains  a  certain  depth  the  water  it  contains 
forms  a  cushion  to  receive  the  blow  of  the  cataract,  and  so 
arrests  the  erosion.      Until  the  vertical  shaft  is  deepened,  the 


CA  J'EJ^XS  AXD   CA  VERN  LIFE. 


109 


water  finds  its  way,  as  in  the  upper 
levels,  horizontally  along  the  surface 
of  the  hard  layer  to  its  next  down- 
ward plunge,  or  until  it  escapes  into 

the  open  streams  of  the  country.  f 

As    this    action   is   repeated    in   a  i^ 

small  or  a  large  way  by  all  the  streams  ^ 

which  enter  the   earth  at  the  bottom  i 

of  the  sink-holes,  it  is  easily  seen  how  | 

o 

the  rock,  for  all  the  depth,  from  the  o 

hiofhest  land  to  the  level  of  the  princi-  5 

pal  rivers,  becomes  in  time  converted  ^ 

into  a  vast  tangle  of  shafts  and  waller-  I 

ies,  so  that  the  mass  often  resembles  a  » 

piece  of  worm-eaten  wood,  the  greater  ? 

part  of  the  strata  having  been  removed  l_ 

by  erosion.     Thus,  within  a  section  of,  i 

say,  ten  square  miles,  and  a  thickness  =» 

of  three  hundred  feet,   in  which   lies  | 

the  Mammoth  Cave,  there  are  proba-  ?" 

bly  in  the  known  and  unknown  galler-  -^ 

ies  more  than  two  hundred  miles  of  s 

ways  large  enough  to  permit  the  pas-  | 

sage  of  a  man,  besides  what  is  proba-  ^ 

bly  a  greater  length  of  smaller  chan-  | 

nels.     Within  the  commonwealth  of  | 

Kentucky,   principally  in  the  subcar-  -^ 
boniferous  limestone,  it  seems  certain 
that  there  is  an  aggregate   length  of 
such   underground    galleries    exceed- 
ing many  thousand  miles.     The  total 


y*-*i' 


1 


I  lO 


ASP  EC  IS    OF  THE  EARTH. 


amount  of  these  underground  passages  would  be  much  greater, 
were  it  not  for  the  deposits  of  stalactitic  matter  which  take 
place  in  them,  and  which,  in  many  parts  of  the  caverns, 
rapidly  work  to  close  the  openings  as  soon  as  they  have 
been  deserted  by  the  channelling  streams. 

The  stalactizing  process  is  brought  about  by  a  modification 
of    the  very  same  action  to  which  the  original   formation  of 


stalactite   Formation   in    Limestone.      (The   arrows    show    the   direction   of  the   movement  of  the   water.) 

the  caverns  is  due — viz.,  to  the  power  of  dissolving  limestone 
given  to  w^ater  by  the  carbonic-acid  gas  which  it  obtains  from 
the  decaying  vegetation.  When  this  water  finds  its  way 
through  an  open  channel,  it  dissolves  the  rock  and  bears  the 
suspended  lime  speedily  away  ;  when,  however,  the  water  has 
to  creep  through  narrow  interstices,  it  advances  very  slowly 
and  in  small  quantities.  Encountering  the  space  of  a  cavern 
in  its  downward  passage,  it  oozes,  drop  by  drop,  through  the 


CA  VERNS  AXD   CA  VERN  LIFE.  1 1 1 

roof  or  into  the  crevices  which  lead  upward  from  it.  As 
there  is  a  constant,  though  slow,  circulation  of  air  through 
these  caverns,  they  are  generally  dry,  and  this  exuding  water 
may  evaporate  without  falling  to  the  floor,  leaving  where  it 
dries  the  various  dissolved  substances  which  it  contains.  In 
this  way  a  slender,  pendant-like  body  begins  to  form  on  the 
ceiling,  and  grows  with  varying  speed  toward  the  floor.  If 
the  incoming  water  is  greater  in  quantity  than  can  be  taken 
up  by  the  air,  it  drops  from  the  hanging  stalactites.  When 
it  strikes,  the  drops  are  shattered.  Evaporation  and  the  loss 
of  the  carbonic  acid  causes  a  still  further  deposition  of  the 
dissolved  matter,  which  crystallizes  in  a  conical  heap,  growing 
upward  to  meet  the  corresponding  descending  cone.  As  the 
water  commonly  penetrates,  not  at  one  point,  but  along  the 
irregular  line  of  crevices,  these  stalactites  are  usually  in 
the  form  of  coalesced  columns,  which  in  time  .form  a  continu- 
ous sheet  which  may  extend  entirely  across  the  space  of  the 
gallery.  If  there  be  many  fissures  in  the  roof,  the  gallery 
may  in  time  become  quite  closed  by  the  conjoined  sheets  of 
stalactitic  material.  This  process  of  depositing  lime  goes  on 
most  actively  in  the  upper  or  oldest  levels  of  the  cavern,  for 
the  reason  that  they  are  nearest  the  surface  and,  therefore,  to 
the  supply  of  the  carbonated  waters  ;  the  lower  levels  of  the 
system  of  caves  are  generally  destitute  of  them,  the  perco- 
lating water  having  found  its  way  into  the  upper  chambers. 
Besides  the  beauty  which  this  stalactitic  material  gives  to 
caverns,  we  owe  to  these  sheets  of  lime  the  preservation  of 
the  various  fossils  which  are  entombed  in  the  caves. 

It  is  interesting  that  so  small  a  circumstance  as  the  speed 
with  wdiich  the  water  flows  through  the  interstices  of  the  rocks 
can  thus  profoundly  affect  the  method  of  its  action.     Where 


r  12 


ASPEC7S   OF  THE  EARTH. 


it  goes  swiftly,  it  excavates  the  caves  ;  where,  moving  slowly, 
it  penetrates  a  large  opening,  it  tends  to  obliterate  the  cavern. 
This  is  but  one  of  many  casgs  in  natural  phenomena  where 
slight  changes  in  circumstances  totally  alter  the  results  of 
processes.* 

There    is    a    curious    destructive    effect    sometimes    exer- 
cised by  the  formation  of  stalactitic  material.      It  occasionally 


A    Stalactite 


Cross-section   of  Stalactite.     (Produced  by 
several   separate  stalactites  growing  together.) 


happens  that  these  accretions,  particularly  those  composed  of 
gypsum,  gather  in  the  crevices  of  the  rock  which  forms  the 
roof  of  the  cavern.  Slowly  accumulating,  these  stalactitic 
masses  wedge  the  rock  apart  and  finally  destroy  the  adhesion 
of    the  blocks  which    constitute    the    roof.      These  separated 

*It  is  commonly  supposed  that  stalactitic  deposits  are  peculiar  to  caverns,  but 
they  may  be  seen  wherever  massive  brick  arches  are  exposed  to  percolating  water  ; 
the  lime  of  the  mortar  pa.sses  into  solution,  and  forms  small  pendent  deposits  exactly 
resembling  those  of  caverns.  Other  substances,  such  as  the  iron  ore  called  limon- 
ite,  also  occasionally  form  beautiful  stalactites  in  the  small  cavities  in  ore-beds 
exposed  to  the  leaching  action  of  percolating  water. 


CA  VERXS  AND   CA  VERX  LIFE. 


113 


masses  fall  to  the  floor  of  the  chamber,  exposing  other  layers 
to  the  same  process  of  dislocation.  In  this  manner  certain 
caverns  which  may  have  been  formed  at  a  low  level  slowly 
rise  upward  towards  the  surface,  until  the  roof  is  carried  to 
the  air  or  until  the  loose  fragments  which  cover  the  floor  have 


Stalactites,  Luray  Cavern.     (Drawn  from  a  photograph   by   C.  H.  Janes.) 

accumulated  to  such  an  extent  that  the  space  is  actually  closed. 
If  the  chamber  of  the  cavern  is  the  seat  of  a  free  running 
stream,  it  may  dissolve  or  break  up  and  bear  away  the  frag- 
ments, and  so  preserve  the  space  of  the  chamber. 

We  have  already  seen  that  in  any  great  district  of  caverns 
we  usually  have  the  underground  spaces  divided  into  distinct 


114  ASPECTS    OF  THE  EARTH. 

floors,  of  which  the  uppermost  was  the  earHest  to  be  con- 
structed. In  such  a  district  the  open-air  rivers  are  constantly 
cutting  their  channels  deeper  into  the  earth,  thus  preparing' 
the  way  for  the  formation  of  yet  lower  levels  of  galleries  ;  at 
the  same  time  the  general  surface  of  the  country  is  wearing 
downward,  only  at  a  slower  rate  than  the  stream-beds  of  the 
open-air  rivers.  If  the  beds  be  nearly  horizontal  (it  is  only  in 
such  districts  that  we  have  very  extensive  caverns),  the  descent 
of  the  upper  surface  is  greatly  restrained  by  the  presence  of 
the  insoluble  layers  which  we  found  to  make  the  throat  of  the 
vertical  shafts,  or  domes.  It  is  often  a  very  long  time,  even 
in  a  geological  sense,  before  the  slight  surface-erosion  acting 
on  a  sink-hole  country  can  wear  through  this  roofing-layer. 
In  time  this  is  accomplished,  and  the  uppermost  chambers  are 
bared  by  the  destruction  of  their  roofs.  Commonly  these 
ruined  g-alleries  are  filled  with  the  debris  of  the  roofs,  in  so 
far  as  they  have  not  previously  been  closed  by  stalactitic 
matter.  It  often  happens  that  the  roofs  do  not  altogether 
fall  in  at  once,  portions  of  the  arches  remaining  standing  for 
ages.  These  constitute  the  "  natural  bridges"  which  are  found 
in  all  cavernous  countries.  Sometimes  the  greater  portion  of 
the  arch  remains,  in  which  case  we  may,  as  in  some  instances 
in  Kentucky,  have  a  momentary  view  of  a  considerable  under- 
ground river,  or  gain  access  to  a  great  system  of  underground 
chambers  which  would  otherwise  be  unknown.  The  Mam- 
moth Cave,  for  instance,  is  entered  by  such  a  tumble  of  the 
roof  of  a  gallery ;  and,  notwithstanding  its  vast  length  of  con- 
nected chambers,  there  is  no  other  practicable  way  into  its 
recesses.  Again,  we  may  find  a  stream  suddenly  vanishing 
beneath  a  dark  archway,  to  reappear  after  a  course  for  many 
miles   underground.      When  a   small    part    of   the   arch    alone 


Natural    Bridge,    Virginia. 


CA  VERXS  AXD   CA  VERX  LIFE.  1 1 5 

remains,  the  structure  takes  the  form  of  the  well-known 
Natural  Brid({e  of  Virofinia. 

In  almost  all  cases  these  natural  bridges  have  free  streams 
beneath  them,  which  bear  away  the  fragments  falling  from 
the  roof,  and  so  permit  the  chamber  to  grow  almost  indefi- 
nitely in  height,  until  the  material  of  the  roof  no  longer  exists. 

When  the  remaining  portion  of  the  arch  is  too  wide  for 
the  term  "natural  bridge"  to  be  suitable,  the  appellation  "nat- 
ural tunnel  "  is  often  applied  to  the  passage.  There  are 
several  passages  of  this  nature  in  the  Eastern  United  States, 
of  which  the  finest  is,  perhaps,  that  near  the  Clinch  River,  in 
Virginia,  where  a  considerable  mountain-stream  fiows  through 
a  vast  arch  for  a  distance  of  over  half  a  mile.  This  natural 
way  is  about  to  be  used  for  the  passage  of  a  railway. 

Let  us  now  turn  to  the  physical  features  of  the  caverns 
other  than  those  which  are  involved  in  their  production. 
Among  these  we  note  the  circulation  of  air  through  the 
caves.  This  is  a  beautiful  and  often  startling  phenomenon. 
If  on  a  hot  summer  day  we  approach  the  lower  exit  of  any 
great  system  of  connected  caverns,  we  are  surprised  by  the 
swift,  cold  wind  which  pours  from  its  mouth  and  inundates 
the  valley  below  with  the  chill  air.  In  Kentucky  this  air 
always  has  the  temperature  of  about  60°  Fahr.,  the  mean  heat 
of  the  upper  earth,  and  thus  often  affords  a  striking  contrast 
to  the  external  temperature.  In  the  summer  season  this  air 
is  derived  from  the  many  small  currents  which  pour  in  through 
the  sink-holes  in  the  high  ground.  It  is  cooled  in  the  vast 
chambers  through  which  it  slowly  moves,  being,  on  the  aver- 
age, some  months  in  its  journey,  and  finally  escapes  at  the 
lower  vents  of  the  cave.  When  the  temperature  of  the  outer 
atmosphere    is   low,    the    current    is    reversed,    entering    then 


Ti6  ASPECTS   OF  THE  EARTH. 

through  the  passages  along  the  rivers,  and  finding  its  exit,  as 
warmed  air,  from  the  myriad  crevices  of  the  uplands. 

In  consequence  of  the  slow  passage  of  this  air  through 
the  cool,  dry  caverns,  where  there  is  almost  no  decomposing 
organic  matter,  it  acquires  a  remarkable  purity,  which  in 
warm  countries  is  only  found  in  the  midst  of  great  deserts. 
We  have  a  sensible  experience  of  this  purity  when,  after  a 
summer's  day  in  a  great  cavern,  we  come  suddenly  into  the 
warm  air  of  a  forest.  For  a  while  the  rank  odor  of  the  vege- 
tation is  most  unpleasant.  We  marvel  that  men  can  live  in 
such  an  impure  element  as  the  air  seems  to  be.  A  more 
satisfactory  proof  of  the  purity  of  the  cavern-air  is  found 
in  the  absence  of  decomposition  in  animal  bodies  exposed  in 
the  inner  recesses  of  caves.  Even  large  animals  fail  to  pass 
through  all  the  stages  of  putrefactive  decay.  A  few  years 
ago  the  body  of  a  young  Indian  was  found  in  a  mummified 
state  in  a  dry  portion  of  one  of  the  caverns  near  the  Mam- 
moth Cave.  The  unhappy  child  had  probably  wandered  away 
into  the  darkness,  and  when  overcome  by  starvation  had  lain 
down  on  a  shelf  of  rock  for  the  sleep  of  death.  Naturally  the 
body  was  much  emaciated  ;  but  the  skin  was  unbroken,  and 
even  the  face  as  little  altered  as  in  a  well-preserved  Egyptian 
mummy. 

These  qualities  of  dryness,  invariable  temperature,  and 
purity  of  the  air  in  the  Mammoth  Cave  have  long  been 
remarked.  At  one  time  a  rude  effort  was  made  to  use  this 
cavern-air  in  the  treatment  of  pulmonary  consumption.  A 
number  of  huts  were  constructed  in  the  main  avenue  of  the 
cave,  which  were  for  a  time  occupied  by  several  persons  suf- 
fering from  this  disease.  As  may  be  imagined,  the  results 
were  most  unhappy.     The  absence  of  sunlight,  combined  with 


CA  VERNS  AND   CA  VERN  LIFE.  1 1  7 

the  sombre  surroundings,  hastened  the  progress  of  a  malady 
which  under  no  circumstances  could  have  been  materially 
helped  by  the  qualities  of  the  air.  This  unhappy  experiment 
has  led  to  a  neglect  of  the  proper  methods  of  using  the 
peculiar  hygienic  qualities  of  the  air  of  caves.  This  can  only 
be  accomplished  by  pumping  the  air  from  the  cavern  to  a 
properly  constructed  sanitarium  on  the  surface  of  the  earth. 
With  the  modern  ventilation-fans  this  can  easily  be  effected. 
Choosing  a  point  where  the  supply  would  be  taken  from  the 
large  chambers  of  a  cavern,  like  the  Mammoth  Cave,  some 
miles  from  the  entrances,  a  very  large  building  could  be  sup- 
plied with  air  of  a  perfectly  uniform  temperature  and  exceed- 
ing purity.  There  can  be  no  question  that  a  hospital  arranged 
for  this  purpose  would  afford  admirable  conditions  for  the 
treatment  of  certain  classes  of  maladies,  especially  where  it 
was  desirable  to  exempt  the  patient  from  the  heat  of  summer, 
from  the  irritating  emanations  of  vegetation,  or  from  malarial 
poisons.'^ 

The  relation  of  primeval  man  to  caverns  was  much  closer 
than  that  of  his  civilized  descendants  is  ever  likely  to  be. 
Before  the  savage  began  to  be  a  constructor  of  dwellings, 
caves  afforded  him  a  natural  and,  in  many  respects,  a  satis- 
factory abiding-place.  At  their  entrances  he  often  found  a 
dry  chamber,  which  could  generally  be  defended  to  advantage; 
the  recesses  of  the  cave  afforded  places  of  refuge  in  case  of 
disaster.  In  the  Old  World  caverns  appear  to  have  been 
much  more  commonly  occupied  as  dwelling-places  than  in  the 
New.  In  any  part  of  Asia  and  Europe  where  the  caverns 
have  been  explored  they  have  given  evidence  of  occupation 

*The  Trocad^ro  Palace  in  Paris  is,  I  believe,  provided  with  a  system  of  pipes 
by  which  the  air  from  the  quarries  beneath  that  city  is  used  for  cooling  the  edifice. 


Il8  ASPECTS    OF  THE  EARTH. 

by  the  ancient  races  of  man.  Some  of  the  most  ancient 
remains  of  the  bodies  and  the  arts  of  those  peoples  have 
been  disinterred  from  beneath  the  stalagmitic  sheets  which 
have  preserved  them."^" 

In    North  America  the    caverns    do  not    appear  to  have 
been,  to  any  extent,  used  as  dwelling-places  by  the  aboriginal 
peoples.     Though  often  resorted  to,  in  but  few  cases  do  they 
appear  to   have  been   continuously  occupied  as  were  those  in 
Europe.     This  is  perhaps  due  to  the  fact  that  the  first  ances- 
tors of  our  Indians    who  came   to  this  country    had  already 
attained 'an  advancement  in  the  arts  which  enabled  them  to 
make  shelters  of  a  more  convenient  sort  than  caverns  afforded. 
About  the  only  considerable  use  which  our  American  Indians 
made  of  these  caves  was  as  burial-places.     They  appear  some- 
times to  have  made  a  rude  disposition  of  the  dead,  or  perhaps 
even   of  their  prisoners  of    war,   by   casting   them   down   the 
shafts  which  lead  to  the  caverns.     More  commonly  they  used 
the  deep  layer  of  fine,  dry  earth  so  often  found  in  the  caverns 
for  deliberate  and  careful  burial.       Lighting  their  path  with 
torches  made  of  cane-Joints  filled  with  tallow,  they  appear  to 
have  wandered  far  into  these  caves,  seeking  for  flints  which 
abound  there,   or  perhaps  trailing   their  escaped   enemies   to 
their    hiding-places.     Occasionally  in  the  innermost  recesses 
of  these  caverns  we  come  to  a  place  where  one  or  more  per- 
sons have  long  lain  concealed,  as  is  shown  by  the  remains  of 
food  or  clothing  which  have  been  left  behind.     Often,  when 
it  appears  as  if  we  had  penetrated  to  some  recess  never  before 
trodden  by  man,  we  find   on  the  cavern-dust  the  footprints  of 
a  savage  predecessor,  which,  though  made  perhaps  centuries 

*  For  a  good  general  account  of  these  cavern-dwellers,  see  Professor  W.  Boyd 
Dawkins's  "  Caves  and  Cave  Hunting." 


CAVERNS  AND   CAVERN  LIFE.  1 19 

ago,  remain  so  fresh  in  this  immutable  realm  that  we  might 
expect  to  encounter  him  on  our  way. 

The  caverns  contain  the  remains  of  many  other  animals 
besides  primitive  man.  In  Europe  many  of  these  caves  are 
singularly  rich  in  vertebrate  fossils.  There  are  two  ways  in 
which  these  fossils  are  brought  into  the  caves.     The  sink-holes 

o 

are,  as  the  farmers  of  Kentucky  have  found  to  their  cost, 
natural  traps  into  which  the  unwary  beast  may  fall.  The 
bones  of  these  creatures  are  swept  on  by  the  current  until, 
becoming  lodged  in  some  crevice,  they  may  be  preserved.  A 
more  frequent  source  of  these  fossil  remains  is  the  habit  of 
certain  beasts  of  prey,  which  leads  them  to- drag  the  bodies  of 
their  victims  into  their  cavern-lairs  that  they  may  devour  them 
at  their  leisure.  The  Old  World  hyena  and  the  jackal,  hav- 
ing been  generally  associated  with  larger  predaceous  beasts, 
such  as  the  lion  and  the  tiger,  were  compelled  to  adopt  this 
habit  to  protect  themselves  in  their  repasts  from  their  stronger 
rivals  in  the  chase.  In  this  way  the  wonderful  accumulations 
of  ofnawed  and  scattered  bones  which  characterize  the  Euro- 
pean  caverns  have  been  brought  together.  In  North  America 
the  carnivorous  mammals,  much  fewer  in  number  than  in  the 
Old  World,  have  never  adopted  the  use  of  the  caves  as  lairs. 
Jackals  and  hyenas  have  never  been  known  here ;  hence  in 
American  caverns  we  have  a  relatively  small  amount  of  bones 
in  the  deposits  of  the  cavern  floors. 

The  living  inhabitants  of  caverns,  those  which  make  these 
regions  of  continuous  darkness  their  abiding-places,  are  nu- 
merous and  of  the  greatest  interest  to  the  naturalist.  Of 
the  several  hundred  species  known  to  students,  by  far  the 
greater  part  belong  to  the  group  of  articulated  animals,  in- 
sects,  and    crustaceans,    these  being  the  forms  which,   of  all 


I20  ASPECTS   OF  THE  EARTH. 

animals,  are  the  most  varied  in  structure  and  best  suited  for 
the  pecuHar  chances  of  Hfe  which  the  caverns  afford.  As  the 
reader  well  knows,  the  great  problem  now  before  science  is  to 
determine  how  far  the  shapes  of  living  creatures  are  deter- 
mined by  the  circumstances  of  the  world  about  them,  and 
how  far  this  determination  has  been  brought  about  through  a 
process  of  selection,  in  a  natural  way,  of  those  varieties  which 
have  some  accidental  special  fitness  for  the  conditions  in  which 
they  live.  Cavern-animals  afford  us  a  capital  bit  of  evidence 
toward  the  solution  of  this  problem.  The  prevailing  close 
affinity  of  their  forms  with  those  which  live  In  the  upper 
world  of  sunshine  and  changing  seasons  shows,  beyond  a 
question,  that  they  are  all  derived  from  similar  forms  which 
once  dwelt  in  the  ordinary  conditions  of  animal  life.  What, 
then,  are  the  effects  arising  from  this  complete  change  in  the 
circumstances  of  these  underground  creatures  ? 

The  facts  are  perplexing  In  their  variety,  and  a  consist- 
ent theory  of  them  by  no  means  well  worked  out,  but  the 
following  points  seem  to  be  well  established,  viz.  :  There  is 
a  manifest  tendency  of  all  gayly  colored  forms  to  lose  their 
hues  in  the  caverns,  and  to  become  of  an  even  color.  This 
may  be  explained  by  the  simple  absence  of  sunshine,  and 
on  it  no  conclusions  can  be  based.  The  changes  of  the 
structural  parts  are  of  more  importance ;  these,  as  might  be 
expected,  relate  mainly  to  the  organs  of  sense.  The  eyes 
show  an  evident  tendency  in  all  the  groups  to  fade  away. 
In  the  characteristic  cavern-fishes  they  have  entirely  disap- 
peared, the  whole  structure  which  serves  for  vision  being  no 
longer  produced.  In  the  cray-fishes  we  may  observe  a  cer- 
tain gradation.  Some  species  which  abound  in  caverns  are 
provided  with  eyes ;  others  have  them  present,  but  so  imper- 


CAVERNS  AND   CAVERN  LIFE.  I2i 

feet  that  they  cannot  serve  as  visual  organs ;  yet  others  want 
them  altogether.  One  species  of  pseudo-scorpion,  as  shown 
by  Professor  Hagan,  has  in  the  outer  world  four  eyes,  while 
in  the  caves  it  has  been  found  with  two  eyes,  and  others 
in  an  entirely  eyeless  condition.  Some  cavern-beetles  have 
the  males  with  eyes,  while  the  females  are  quite  without 
them.  As  a  whole,  the  cavern-forms  exhibit  a  singular  tend- 
ency of  the  visual  organs,  not  only  to  lose  their  functions, 
but  also  to  disappear  as  body-parts.  At  the  same  time  there 
is  an  equal,  or  even  more  general,  development  of  the  anten- 
nae and  other  organs  of  touch  ;  these  parts  become  consid- 
erably lengthened,  and  apparently  of  greater  sensitiveness,  a 
change  which  is  of  manifest  advantage  to  the  individual. 

It  is  probable  that  the  organic  species  which  inhabit  our 
caverns  have  generally  been,  for  some  geological  periods, 
existing  in  the  peculiar  conditions  of  caves.  Thus,  in  the 
Mammoth  Cave  district  of  Kentucky,  or  the  neighboring 
fields  of  Tennessee,  the  present  levels  of  the  subterranean 
chambers  have  probably  inherited  their  animal  life  from 
stages  or  stories  of  the  caves  which  have  since  been  destroyed 
by  erosion.  In  some  regions  the  geologists  can  show  that 
above  the  level  of  the  existing  caverns,  for  the  height  of  a 
thousand  feet  or  more,  in  what  is  now  mid-air,  caves  probably 
have  existed  in  former  geological  periods,  which  have  slowly 
been  worn  down  by  atmospheric  decay.  On  these  former 
levels  of  caverns,  through  geological  ages,  the  organic  spe- 
cies have  been  undergoing  variations  which  have  gradually 
changed  them   from  their  ancestral  forms. 

The  bearing  of  these  changes  on  the  Darwinian  theory 
is  as  follows  :  That  hypothesis,  at  least  in  the  form  in  which 
it  is  generally  held,  considers  that  the  important  changes  in 


122  ASPECTS   OF  THE  EARTH. 

organic  species  are  the  results  of  a  successful  struggle  for 
existence  of  creatures  possessed,  through  a  chance  variation, 
of  some  slight  advantage  over  their  kindred.  The  difficulty 
which  the  objectors  to  this  view  find  in  their  way  is  that,  in 
the  perplexing  variety  of  conditions  of  the  outer  world,  it  is 
wellnigh  impossible  to  say  that  this  or  that  peculiarity  is  not 
of  great  advantage  under  some  circumstances,  the  selective 
effects  of  which  are  not  manifest  to  the  observer.  1  he 
admirable  feature  in  this  great  natural  experiment,  which  is 
brought  about  by  the  imprisonment  of  organic  forms  in  caves, 
is  that  it  very  much  limits  the  speculation-breeding  confusion 
of  the  outer  world.  Thus  it  at  once  becomes  clear  that  the 
loss  of  eyes  cannot  well  be  the  direct  result  of  any  selective 
action  ;  it  must  arise  from  the  immediate  influence  of  the 
darkness.  It  is  scarcely  less  clear  that  the  corresponding 
development  of  the  tactile  organs  must  be  due  to  something- 
else  than  selection  ;  for  the  cavern-life,  at  best  scanty  in  any 
one  cave,  cannot  be  conceived  to  afford  the  conditions  of 
strenuous  battle  which  exist  in  the  overground  world.  It  must 
not  be  supposed  that  this  evidence  goes  to  overthrow  the 
fundamental  propositions  of  the  Darwinian  hypothesis  ;  it 
only  shows  that  we  must  carefully  limit  the  action  of  the  "sur- 
vival of  the  fittest,"  and  that  we  must  be  prepared  to  allow  a 
large  share  in  the  development  of  organic  forms  to  forces 
which  have  nothing  to  do  with  selection, — to  the  innate  or- 
ganic impulses,  or  to  the  immediate  action  of  environment. 

A  word  concerning  the  geographical  distribution  of  this 
group  of  superficial  caverns,  and  we  shall  have  done  with  this 
division  of  our  subject.  So  far  as  the  present  writer  has  been 
able  to  observe  American  caverns,  they  have  been  limited  to 
the  regions  south  of  the  vast  field  occupied  by  the  ice  sheet  of 


Brand's   Cascade,    Luray   Cavern.     (Drawn   from   a  photograph    by   C.  H,  Janes.) 


CAV£R.\S  AM)   CAVERN  LIFE.  123 

the  last  glacial  period.  But  in  New  York  and  elsewhere  there 
are  some  small  caverns  which  were  within  that  field  of  ice. 
It  is  an  important  task  for  students  to  find  whether  these 
caverns  existed  before  the  ice-period,  or  whether  they  have 
been  formed  since  that  time.  If  they  survived  the  glacial 
period,  as  seems  likely,  then  they  afford  valuable  evidence  to 
show  that  the  ice  did  not  wear  away  as  great  a  depth  from  the 
surface  of  the  country  as  is  commonly  supposed. 

The  second  group  of  caves  exhibits  a  certain  general 
resemblance  to  those  just  described.  These  are  the  caverns 
which  have  been  formed  by  hot  waters  on  their  way  to  the 
surface,  where  they  emerge  as  hot  springs,  or  geysers.  These 
hot  spring-waters  are  in  the  main  rain-water  which  has  pene- 
trated to  great  depths  below  the  surface,  and  become  heated 
by  the  internal  temperature  of  the  earth  ;  this  rain-water  is 
more  or  less  commingled  with  the  old  sea-waters  which  were 
built  into  the  strata  through  which  it  has  passed  in  its  slow 
underground  journey.  Unlike  the  cavern-making  streams 
which  excavate  the  superficial  caves  just  before  described, 
these  spring-waters  rising  from  the  depths  of  the  earth  do 
their  work  by  ascending  currents,  w4th  no  direct  help  from 
gravitation  ;  their  action  is  therefore  not  mechanical  or  ero- 
sive, but  chemical  or  corrosive.  They  do  not  tend  to  excavate 
a  succession  of  galleries,  one  above  the  other,  but  work  to 
open  single  channels  of  escape.  When  in  their  upward  path 
they  encounter  deposits  of  limestone,  they  rapidly  enlarge  the 
spaces  through  which  they  flow,  making  great  chambers  where 
the  rock  is  soluble,  connected  by  narrower  fissures  through 
the  less  soluble  parts  of  the  deposit.  The  solvent  power 
of  the  water  is  in  part  due  to  the  carbonic-acid  gas  it  ob- 
tained from  the  decayed  vegetation  before  it  started  on  its 


124  ASPECTS   OF  THE  EARTH. 

downward  journey,  and  in  part  to  the  further  contribution  of 
this  and  other  gases  given  to  it  by  the  various  decompositions 
going  on  in  the  heated  depths  of  the  earth.  The  elevated 
temperature  of  the  water  also  aids  its  work  of  corrosion. 

In  the  superficial  cold-water  caves,  as  we  have  already  seen, 
the  caverning  cannot  go  on  at  depths  below  the  general  levels 
of  the  main  streams  of  the  district  in  which  the  caverns  lie  ; 
but  in  these  hot-spring  caves  the  excavation  can  go  on  at 
depths  of  miles  below  the  surface.  Springs  of  this  nature  are 
particularly  characteristic  of  mountainous  districts,  where  the 
strata  lie  at  high  angles.  They  are  also  found  in  regions 
where  volcanoes  are  or  have  recently  been  in  action.  It  is 
easy  to  see  that  either  one  of  these  conditions  favors  the 
development  of  such  hot-water  caverns.  In  the  mountainous 
districts  this  is  effected  by  the  presence  of  rifts  in  the  rock,  or 
of  highly  inclined  porous  strata,  which  conduct  the  surface- 
waters  to  great  depths.  In  these  depths  the  rocks  are  highly 
heated  by  the  internal  temperature.  Partaking  of  this  internal 
heat,  the  water  passes  upward  through  any  chance  way  lead- 
ing to  the  surface.  In  volcanic  districts  the  water,  after  a 
much  shorter  downward  journey,  may  find  itself  in  contact 
with  masses  of  lava  or  rocks  which  are  at  a  high  temperature 
because  they  have  recently  been  traversed  by  volcanic  fires. 

We  note  that  at  the  mouth  of  these  hot  springs  and 
geysers,  the  waters  of  which  have  passed  through  limy  rocks, 
there  is  a  very  extensive  deposit  of  lime,  which  is  laid  down 
at  once  as  soon  as  the  temperature  of  the  solution  falls  by 
exposure  to  the  open  air.  These  hot-spring  deposits  often 
constitute  very  extensive  accumulations  of  rocky  material ; 
as,  for  instance,  in  the  Yellowstone  district.  They  afford  a 
rough  indication  of  the  cavern-making  power  of  the  waters  on 


CAVERNS  AND   CAVERN  LIFE.  1 25 

their  way  to  the  surface.  It  must,  however,  be  remembered 
that  only  a  portion,  probably  much  less  than  half  of  the  dis- 
solved rock,  is  laid  down  at  the  mouth  of  the  spring  ;  a  larger 
part  passes  to  the  rivers,  and  thence  to  the  sea. 

Our  knowledge  of  these  hot-spring  caverns  is  not  alto- 
gether theoretical.  It  happens  that  the  abandoned  channels 
of  these  springs  are  often  the  seat  of  important  deposits  of 
the  precious  metals,  which  has  led,  in  this  country,  to  their 
becoming  the  seat  of  extensive  mining  operations.  There 
are  at  least  half  a  dozen  extensive  mines  which  have  followed 
these  cavern-deposits  in  the  district  of  the  Rocky  Mountains  ; 
it  is  likely  that  there  are  very  many  others  which  await  the 
explorer.  The  origin  of  these  mineral  deposits  is  probably  as 
follows  :  After  the  heated  waters  have  excavated  the  cav- 
erns, and  ceased  to  flow  with  their  original  speed,  the  chasms 
become  the  place  of  deposit  of  mineral  matters  which  are 
brought  into  them  by  the  creeping  movement  of  waters  mov- 
ing up  from  below  or  oozing  out  from  the  rock  on  the  sides 
of  the  cavity.  While  the  stream  of  water  flowed  rapidly 
upw^ard  there  was  no  chance  for  the  chambers  to  become 
filled  with  mineral  materials  ;  as  soon  as  the  currents  were 
arrested,  the  mineralizing  process  would  begin.  The  reader 
will  note  the  likeness  which  exists  between  this  process  and 
that  by  which  the  abandoned  upper  chambers  of  the  cold- 
water  or  superficial  caverns  are  filled  with  stalactitic  material 
by  the  creeping  into  the  chambers  of  water  charged  with  dis- 
solved substances  ;  the  only  important  difference  being  that  in 
the  superficial  caverns  the  water,  being  cold,  can  only  take 
out  of  the  rock  and  convey  into  the  gallery  the  very  soluble 
limy  materials,  while  in  the  deeper  caverns  the  heated  water 
can  transfer  many  less  soluble  mineral  substances. 


126  •  ASPECTS    OF  THE  EARTH. 

In  the  cordilleran  district  of  North  America,  hot  springs 
are  still  of  common  occurrence,  and  have  been  more  abundant 
in  former  eeoloeical  aofes  since  these  mountains  were  formed. 
The  result  is  that  the  mineral  deposits  formed  in  such  cham- 
bers are  of  considerable  economic  importance  as  well  as  of 
much  scientific  interest'.  It  seems  probable  that  a  careful 
search  of  this  reeion  for  accumulations  of  this  nature  will 
hereafter  develop  a  great  number  of  deposits  of  this  class 
which  are  as  yet  unknown.  There  are  several  reasons  why 
cavern  lodes  are  likely  to  escape  observation.  In  the  first 
place,  while  in  ordinary  fissure  veins  the  lode  has  a  great 
linear  extension,  and  thus  is  apt  to  outcrop  at  many  points; 
then,  in  the  case  of  fissure  veins,  the  material  filling  the  cavity 
is  commonly  either  harder  or  softer  than  the  surrounding 
rocks,  and  so  the  lode  appears  either  as  a  ridge  or  a  trough 
upon  the  surface  of  the  country ;  furthermore,  in  the  com- 
moner class  of  veins,  the  mineral  matter  is  likely  to  be  in 
a  compact  form,  which  remains  in  the  form  of  conspicuous 
fragments  on  the  surface  and  so  affords  a  "  blossom,"  or  trail, 
which  the  prospector  can  readily  perceive ;  in  cavern  mines 
the  deposit  will  appear  on  the  surface  in  the  form  of  a  small 
opening,  which  is  readily  concealed  by  superficial  material. 
The  substance  filling  the  chamber  is  in  many  cases  of  a  very 
soft  nature,  and  so  completely  disintegrated  as  to  leave  no 
superficial  indication  of  the  presence  of  the  ore  masses  which 
lie  below.  I  am  satisfied  that  a  careful  search  for  cave  mines 
is  likely  to  prove  in  a  large  measure  profitable  to  skilful  and 
enterprising  prospectors.  I  therefore  venture  to  indicate  a 
few  conditions  which  may  possibly  lead  to  a  successful  search 
for  this  class  of  deposits. 

Cave   lodes    commonly  occur  in   regions  where  limestone 


CAVERNS  AND   CAVERN  LIFE.  12 J 

deposits  of  considerable  thickness  are  found.  They  most 
generally  appear  either  where  such  rocks  are  riven  by  faults 
or  where  the  beds  dip  at  tolerably  steep  angles.  Hot-spring 
deposits,  such  as  excavate  caverns  and  charge  the  chambers 
with  mineral  deposits,  generally  form  considerable  accumula- 
tions of  what  we  may  compare  to  stalactitic  matter  on  the 
surfaces  near  the  point  of  escape.  If  the  hot  spring  has  not 
long  ceased  to  flow,  it  may  be  possible  to  identify  the  site  of 
these  mines  by  the  remains  of  such  accumulations,  which  may 
thus  give  the  clew  to  the  occurrence  of  hidden  cavern  cham- 
bers. Inasmuch,  however,  as  these  accumulations  of  "sinter" 
are  generally  thin,  they  may  have  been  destroyed  by  superfi- 
cial agents  of  erosion,  the  rain,  the  streams,  or  the  action  of 
ancient  glaciers  ;  therefore,  their  absence  should  not  be  taken 
as  indication  that  cave  deposits  are  wanting  in  a  given  dis- 
trict. 

That  cavern  mines  are  abundant  in  the  Rocky  Mountains, 
though  as  yet  but  few  have  been  found,  is  shown  by  the 
curious  nature  of  the  chances  by  which  they  have  been  dis- 
covered. Thus,  one  of  the  most  interesting  of  these  deposits, 
a  lode  which  has  proved  quite  valuable,  was,  I  am  credibly 
informed,  discovered  in  the  following  curious  manner  :  A 
miner  dreamed  that,  if  he  dug  into  a  limestone  cliff,  he  would 
find  an  ore  deposit.  He  proceeded  to  run  a  tunnel  into  the 
unprofitable  looking  rock,  and  at  the  end  of  a  long  and  profit- 
less experience  found  himself  in  a  chamber  which  contained  a 
considerable  deposit  of  ferruginous  clay  which  held  a  profita- 
ble amount  of  Qrold.  AlthouMi  a  sincrle  instance  of  this  sort 
does  not  prove  much,  it  seems  to  indicate  that  the  limestone 
districts  of  the  Cordilleras,  especially  those  portions  of  the 
country  which  have  been  the  seat  of  hot-spring  action,  should 


128  ASPECTS   OF  THE  EARTH. 

receive  more  attention  than  has  been  given  to  them  by  mining 
prospectors. 

It  is  probable  that  the  class  of  cave  mines  exist  in  other 
countries,  but,  owing  to  the  fact  that  the  combination  of  hot 
springs  with  thick  limestone  deposits  of  a  multifarious  charac- 
ter is  seldom  found,  they  are  probably  among  the  rarer 
classes  of  mineral  veins,  and,  therefore,  have  not  been  much 
studied  by  mining  geologists  or  noticed  by  ordinary  pros- 
pectors. 

In  mountainous  countries,  where,  by  the  folding  and  shov- 
ing about  of  the  rocks,  the  strata  have  been  subjected  to  rend- 
in  o-  strains,  we  find  another  class  of  subterranean  crevices, 
which  may  be  confounded  with  the  hot-spring  excavations. 
These  fault-fissures  contain  by  far  the  largest  number  of 
mineral  deposits  which  are  explored  for  the  precious  metals. 
They  are  generally  in  the  form  of  very  long  cracks,  which 
extend  horizontally  and  vertically  for  great  distances,  but  are 
usually  very  limited  in  width.  A  tolerable  idea  of  their  form 
and  nature  may  be  gained  by  studying  the  fissures  in  walls 
which  arise  from  the  settlement  of  their  foundations,  and 
those  which  form  in  timber  from  the  drying-out  of  the  sap. 
We  sec  that  the  crevices  in  walls  are  due  to  the  down-slipping 
of  the  materials  on  one  side  of  the  fracture,  thus  making  a 
very  irregular  fissure  ;  while  in  the  fissured  wood  there  is  no 
movement  of  the  two  sides  past  each  other,  the  walls  simply 
gaping  apart  without  other  dislocation.  In  most  cases  both 
these  classes  of  earth  fissures  are  filled  with  mineral  matters 
sweated  out  from  the  side  walls,  or  brought  up  from  below  as 
fast  as  the  crevices  are  produced  ;  so  that  hardly  any  space  is 
ever  formed,  or  if  formed  is  quickly  filled  with  vein-matter. 
But  where  the  rocks  are  dry  these  rents  may  remain  unfilled. 


CA  VERAS  AAD   CA  VERN  LIFE.  I  29 

In  parts  of  the  Rocky  Mountain  mining-regions  the  explorer 
occasionally  finds  his  drills  penetrating  one  of  these  cavities. 
Breaking  through  the  wall,  the  space  may  be  found  to  have  a 
width  of  several  feet  and  an  indefinite  extension  downward 
and  on  either  side.  Sometimes  the  walls  are  thinly  coated 
with  a  vein-deposit,  formed  before  the  waters  abandoned  the 
cavity ;  in  other  cases  they  remain  bare,  as  when  they  were 
first  rent  apart.  Even  the  hardy  miners,  accustomed  to  the 
mysteries  of  the  underground,  recoil  from  the  risks  of  explor- 
ing the  strange  depths  of  these  fissures.  There  seems  to  be 
little  chance  that  they  may  lead  to  mineral  deposits  of  value, 
for  the  reason  that  they  have  never  been  the  seat  of  the  actions 
which  build  such  deposits.  The  only  use  the  miner  makes  of 
them  is  to  cast  the  rubbish  of  his  excavations  into  their  cavi- 
ties. It  is  greatly  to  be  desired  that  some  of  these  fissures 
should  be  thoroughly  explored,  for  thereby  we  are  likely  to 
gain  much  knowledge  as  to  the  conditions  of  fault-chasms 
before  they  become  the  seat  of  mineral  deposits. 

It  has  already  been  said  that  the  caverns  scoured  out  by 
heated  waters  have  frequently  been  confounded  with  these 
dislocation-fissures.  There  is  good  reason  for  this  confusion  ; 
for  the  hot  springs,  on  their  way  to  the  surface,  generally 
make  avail  of  such  fractures,  enlarging  them,  when  they  pass 
through  limestone-deposits,  into  the  spacious  openings  of  cav- 
erns, and  occasionally  filling  with  mineral  deposits  the  parts 
of  the  fissure  through  which  the  water  does  not  move  with 
speed.  We  may  therefore  amend  our  statement  concerning 
the  hot-spring  caves,  by  saying  that  the  caverns  of  this  group 
are  generally  local  enlargements  of  fissures  when  they  extend 
through  limestones.  In  the  ordinary  fissure-vein  deposits  we 
may  find   traces  of   caverning,  even  in  rocks  which  are  rnuch 


1 2,0 


ASPECTS   OF  THE  EARTH. 


more  resistant  to  the  action  of  heated   waters    than   are   the 
limestone-deposits. 

We  have  now  to  consider  a  class  of  caves  which  are  the 
result  of  water-action,  but  of  water  operating  in  an  entirely 
different  way  from  the  underground  streams.  The  caverns  of 
this  our  last  division  of  water-made  caves  are  formed  by  the 
beating  of  the  waves  against  the  cliff-bordered  shores  of  lakes 


Sea-shore  Cave.     (Showing  action   of  the  sea  at  different  levels.) 


and  seas.  The  reader  has  probably  seen  some  examples  of 
this  peculiar  form  of  caverning,  or  at  least  is  familiar  with  the 
blow  which  the  waves  strike  against  the  shore.  At  the  outset 
let  us  gain  an  idea  of  the  wa\'  in  which  this  forte  of  the  waves 
is  committed  to  them,  and  by  their  motion  applied  to  the  land. 
It  is  well  known  that  this  force  is  due  to  the  friction  of 
the  wind  aq-ainst  the  surface  of  the  water,  causincr  the  water 
to  oscillate  in  somewhat  the  same  way  in  which  the  fiddle- 
strinof  vibrates  when  the  bow  is  drawn  over  its  surface.      In 


Rafe's   Chasm,    near   Gloucester,    Massachusetts. 


CAVERNS  AND   CAVERN  LIFE.  131 

this  manner  the  energy  which  was  in  the  wind  comes  in  part 
to.  be  given  to  the  water,  where  it  is  manifested  in  the  force 
with  which  the  wave  moves  forward,  and  the  height  through 
which  the  water  is  swung  in  its  up-and-down  motion.  Thus 
the  energy  of  the  winds,  over  a  wide  field  of  the  ocean,  is 
committed  to  the  waves  and  sent  against  the  land,  where  it  is 
expended  in  the  blows  they  strike.  Owing  to  the  swiftness  of 
motion  of  the  waves,  they  apply  a  prodigious  force  against 
the  obstacles  of  the  shore.  Their  velocity  of  movement  is 
sometimes  as  much  as  sixty  feet  per  second,  and  the  pressure 
they  apply  to  any  fixed  object  they  encounter  exceeds  six 
thousand  pounds  to  the  square  foot  of  resisting  surface,  or 
perhaps  one  hundred  times  the  force  of  a  storm-wind  which 
produces  this  wave-motion. 

Where  the  wave  meets  a  steep  cliff  of  compact  rock,  at 
whose  base  the  sea  is  deep,  this  pressure,  though  great,  may 
have  little  disruptive  power  ;  but  where  the  water  is  shallow, 
and  there  are  fragments,  which  various  chances  have  sepa- 
rated from  the  shore,  lying  on  the  bottom,  it  tosses  these  with 
great  force  against  the  opposing  wall.  Stones  three  feet  in 
diameter,  though  weighing  over  a  ton,  are  sometimes  hurled 
against  the  cliff  as  swiftly  as  a  strong  arm  can  throw  a  pebble. 
The  rebound  due  to  the  elasticity  of  the  rock  and  the  reflux 
of  the  wave  rolls  the  stone  away  from  the  point  where  it 
strikes,  so  that  a<jain  and  as^ain,  several  times  a  minute,  with 
each  incoming  wave,  the  blow  is  repeated,  until  the  sea  be- 
comes quiet  or  the  stone  is  ground  to  powder.  In  this  way 
every  rocky  escarpment  whose  base  rests  in  shallow  water 
is  constantly  undermined,  and  the  overhanging  fragments  fall 
down,  to  be  in  turn  used  to  batter  the  base  of  the  cliff. 

It  is  almost  certain  that  the  resisting  power  of  this  rocky 


132 


ASPECTS    OF  THE  EARTH. 


wall  of  the  shore  will  very  much  vary  from  place  to  place 
along  its  line.  Differences  in  actual  hardness  will  favor  or 
hinder  the  assault  of    the  sea,   causing  the  line  to   have  the 


-  ^^"«v'«i"'>«!|;^fl^'J5^  ■ 


Chasm    worn   through    by  the   Sea,  Azores. 


combined  salient  and  re-entrant  angles — to  borrow  a  term 
from  the  art  of  fortification — which  give  picturesqueness  to 
the  rock-bound  shores  of  the  ocean.  On  each  of  these  small 
re-entrant  angles  the  sea  has   more  cutting-power  than  on  the 


CAVERNS  AND   CAVERN  LIFE.  133 

headlands,  at  least  until  the  bay  extends  some  distance  into 
the  land  ;  partly  for  the  reason  that  in  this  bay  the  waves  are 
somewhat  heaped  up  by  the  convergence  of  the  shores,  but 
mainly  because  the  fragments  of  rock  torn  from  the  headlands 
are  swept  into  these  pockets,  and  thus  provide  the  waves  with 
the  armament  with  which  they  do  their  effective  work.  Im- 
prisoned in  these  contracted  bounds,  the  bowlders  cannot  be 
dragged  out  by  the  waves  into  deep  water,  and  thus  the  sup- 
ply is  generally  sufficient  to  insure  a  constant  cutting-action 
as  long  as  the  waves  are  high. 

From  the  apex  of  this  re-entrant  angle,  where  the  blow  of 
the  wave-hurled  stones  is  most  effective,  a  cavern  is  apt  to 
extend  into  the  cliff.  It  is  generally  narrow,  and  thus  the 
overlying  rock  is  readily  supported  for  the  width  of  the  arch. 
It  may  be  driven  in  for  a  distance  of  some  hundreds  of  feet 
before  the  friction  of  the  waves  on  its  sides  exhausts  their 
power,  or  the  pressure  of  the  air,  which  is  driven  before  the 
piston  of  water  as  it  rushes  in,  filling  the  whole  space  of  the 
crevice,  hinders  the  action  of  the  blow.  When  these  caves 
are  excavated  in  rock  containing  many  rifts,  a-s  do  most  of 
those  along  our  American  shores,  the  constant  jarring  of  the 
waves  and  the  action  of  frost  are  apt  to  tumble  the  roof 
into  the  space  below.  In  this  case  the  crevice  assum.es  the 
form  of  a  chasm,  or  a  spouting-horn.  The  only  really  fine 
sea-caves  which  the  present  writer  has  seen  along  the  Amer- 
ican coast  are  in  the  Magdalen  Islands  of  the  Gulf  of  St. 
Lawrence,  and  the  other  shores  of  that  noble  sea,  where  rel- 
atively soft  rocks,  with  few  disorganizing  rifts,  are  open  to 
the  assaults  of  the  waves.  In  Europe,  because  of  the  much 
greater  extent  of  shores  of  soft  and  tolerably  massive  rocks, 
these  sea-caves  are  much  more  numerous  and  far  more  beau- 


134 


ASPECTS    OF  THE  EARTH. 


tifiil  than  any  of  this  country.  They  are  particularly  abun- 
dant about  the  Mediterranean.  The  reader  is  likely  to  be 
familiar  with  the  famous  Blue  Grotto  of  Capri,  which  is  an 
excellent  type  of  these  sea-caves,  though  it  probably  has 
been  somewhat  modified  by  art.  A  better  known  and  much 
more  beautiful  variety  of  caverns  occurs  where  columnar  ba- 
salts,  with   the  columns  in   a  vertical    position,    face  the  sea- 


The   Blue   Grotto,   Island   of  Capri. 

waves,  as  at  Staffa,  an  island  on  the  west  coast  of  Scotland. 
Here  the  jointing  of  the  several  columns  enables  the  sea  to 
rend  them  to  advantage,  while  a  rock  of  a  different  character 
serves  as  a  covering  for  the  cave.* 

The  last  group  of  caverns  which  are   in   any  way  due  to 
the    work   of   water    is  the   picturesque   though   unimportant 

*  It  has  recently  been  claimed  that  these  Scotch  basaltic  caves  were  artificial 
works,  excavated  to  serve  as  harbors  at  some  unknown  time  in  the  past  and  by 
some  unknown  people.  Notwithstanding  the  artificial  look,  clue  in  the  main  to  the 
masonry-like  character  of  the  columns  of  basalt,  there  is  no  doubt  in  the  minds  of 
geologists  that  they  are  the  work  of  the  waves  alone. 


CAVERNS  AND   CAVERN  LIFE.  135 

group  of  grottoes  known  in  the  Alleghany  Mountains  as  rock- 
houses.  These  interesting  recesses — hardly  to  be  termed  cav- 
erns, for  they  never  penetrate  the  cliffs  beyond  the  light  of 
day — abound  in  Kentucky,  Virginia,  and  Tennessee,  and  are 
usually  limited  to  the  escarpments  or  outcrop-cliffs  of  the 
millstone-erit,  a  thick  formation  of  sands  and  conelomerates 
which  underlies  the  true  coal-measures.  The  hardness  of  this 
formation  varies  greatly.  There  is  often  a  very  resisting 
stratum  above  a  bed  where  the  rock  is  so  soft  that  it  may  be 
crumbled  by  the  fingers.  When  this  softer  portion  becomes 
wet,  and  then  exposed  to  severe  cold,  its  outer  surface  often 
becomes  converted  into  sand,  which,  as  soon  as  the  frost 
leaves  it,  falls  to  the  floor.  This  sand  is  caught  up  by  the 
wind  and  blown  away  ;  but  before  it  escapes  from  the  recess 
it  is  much  beaten  against  the  soft  walls,  still  further  assisting 
the  process  of  decay.  In  this  manner  the  grotto  is  enlarged, 
to  the  point  where  the  overhanging  rock  is  no  longer  sup- 
ported and  falls  across  the  front  of  the  arch.  It  is  common 
to  find  these  recesses  with  an  overhanging  roof  projecting 
from  thirty  to  fifty  feet  beyond  the  innermost  part  of  the 
grotto.  This  soft  sandstone,  the  excavation  of  which  forms 
the  "rock-house,"  is  often  penetrated  by  interlaced  harder 
lines,  where  the  sand  has  been  cemented  by  oxide  of  iron 
which  has  penetrated  along  the  joints.  When  the  walls  have 
long  been  scoured  by  the  wind-swept  sand,  these  harder  parts 
stand  out  from  the  wall,  forming  a  singular  and  beautiful  fret- 
work, resembling  in  its  decorative  effect  the  arabesque  figures 
of  Moorish  ornamentation.  The  rock-house  type  of  grotto 
in  the  Eastern  United  States  is  almost  altogether  limited,  so 
far  as  the  present  writer's  observations  go,  to  the  millstone- 
grit,  though  they  scantily  occur  in  some  of  the  sandstones  of 


136  ASPECTS    OF   THE  EARTH. 

the  overlying  true  coal-measures.  But  in  the  millstone-grit, 
from  Pennsylvania  south  to  Alabama,  they  so  abound  that 
for  almost  the  whole  distance,  where  the  edge  of  this  grit 
is  exposed,  there  is  hardly  a  mile  where  there  is  not  a  com- 
fortable shelter  from  a  thunder-shower,  where  the  sheep  find 
protection  in  winter  storms,  and  the  lion-spiders  make  their 
curious  traps  of  sand.  This  continuous  undercut  cliff  shows 
us  how  the  topography  of  a  country  is  dependent  on  the 
structure  of  the  rocks  which  underlie  its  surface,  and  how  the 
physical  conditions  of  any  one  stage  of  the  earth's  history 
continue  for  all  time  to  have  a  permanent  influence  on  its 
aspect.  The  millstone-grit  deposit  was  formed  at  a  stage  in 
the  earth's  history  when  great  quantities  of  sand  and  pebbles 
were  swept  about  by  strong  currents,  and  rapidly  built  into 
beds  which  differ  greatly  in  their  coherence.  It  generally 
happens  that  the  upper  layers  of  this  formation  are  much 
harder  than  the  lower  ;  hence  the  steep  and,  often,  overhang- 
ing wall  along  its  outcrop. 

In  the  Rocky  Mountains  this  peculiar  structure  occurs  in 
later  stages  of  the  geologic  periods,  and  affords  many  noble 
grottoes  of  the  rock-house  type.  In  both  the  eastern  and 
western  districts  these  overhanging  cliffs  were  more  frequently 
used  by  the  Indians  for  dwelling-places  than  the  true  caves. 
In  Kentucky  they  were,  apparently,  in  some  cases  the  seats 
of  a  tolerably  permanent  settlement,  as  is  shown  by  the 
occasional  mortars,  for  grinding  corn,  which  the  people  had 
excavated  in  the  hard  masses  of  sand-stone  near  the  shelter- 
ing arches  of  rock.  In  the  Rocky  Mountains  the  aborigines 
built  considerable  masonry  edifices  in  these  grottoes,  contriv- 
ing them  so  that  they  might  serve  at  once  for  dwellings  and 
as   defences   against   attack.      Except   that   these   holds   were 


CA  J^£J^.YS   AXD    CA  J'ERX  LIFE. 


^Z7 


generally  destitute  of  water,  they  afforded  excellent  places  of 
defence,  as  they  were  assailable  on  but  one  face,  and  that 
often  very  easily  defended. 

We  now  have  to  consider  the  last  and  smallest  group  of  cav- 
erns— those  which  are  formed  by  the  draining  out  of  lava  from 
beneath  an  arch  or  roof  which  the  solidification  of  the  fluid 


Cave-dweilings,    Nevada.      (Showing   "Rock    House"    type  of  caverns.) 

rock  has  formed.  It  is  hardly  necessary  to  show  the  reader 
how  exceptional  this  group  is;  how  it  is  limited  to  volcanic 
countries,  and  even  there  is  of  slight  importance,  if  we 
measure  that  importance  by  the  number  and  extent  of  the 
underground  spaces  which  come  into  the  class.  Although 
this  group  of  caverns  is  limited  in  number,  it  constitutes  some 
of  the  most  interestincr,  as  well  as  the  least  known,  of  the  sub- 


138 


ASPEC7S   OF  THE  EARTH. 


terranean  spaces  of  the  earth.     The  commonest  way  in  which 
volcanic  caverns    are    formed  is  as  follows  :    When  the  lava 


Cave   under    Lava   Crust,  Sandwich    Islands.     (Formed   by   the   flowing  away   of   lava  fronn 
beneath   a   hardened   crust.) 

contained  in  a  crater  remains  for  some  time  at  one  level,  it 
freezes,  or  solidifies,  as  a  thick  sheet  across  the  floor  of  the 
cup-shaped  cavity.      After  it  has  become  firm,  the  lower-l^'ing 


CAVERNS  AND   CAVERN  LIFE.  139 

fluid  rock  may,  as  the  gases  which  urged  it  upward  leak  out 
from  the  crevdces  of  the  sohd  crust,  slowly  subside  into  the 
depths  of  the  earth,  leaving  spaces  of  irregular  form  and, 
often,  of  vast  extent.  If  the  volcano  remains  long  dormant — 
some  of  them  are  quiet  for  thousands  of  years — the  rain-water 
gathered  in  the  crater  may  fill  these  lava-caverns.  At  first 
it  is  hot  and  charofed  with  acids,  which  make  it  unfitted  for  the 
habitation  of  animals,  but  in  time  the  temperature  is  lowered 
and  the  water  purified.  It  sometimes  happens  that  these 
great  cisterns  of  water  become  the  dwelling-place  of  fishes, 
as  well  as  of  more  lowly  organized  creatures.  If  now  the  vol- 
cano resumes  its  activity,  this  water,  commingled  with  the 
pulverized  lava,  termed  ash,  and  containing  an  abundance  of 
dead  animals,  may  be  poured  over  the  lip  of  the  crater,  or 
be  tossed  into  the  air,  inundating  the  neighborhood  with  a 
muddy  torrent. 

Another  form  of  lava-caves  is  found  outside  of  the  crater, 
where  the  lava-streams  pour  down  the  slopes  of  the  cone. 
These  streams  naturally  flow  in  the  deep  and  narrow  torrent- 
cut  valleys  which  so  frequently  seam  the  sides  of  the  volcanic 
elevations.  At  first  the  lava  may  flow  with  considerable 
swiftness  ;  but  as  it  becomes  cooler  the  surface  curdles,  like 
flowing  pig-iron,  while  the  mass  below  retains  its  original 
fluidity.  This  hardening  of  the  surface  progresses  until  the 
roof  is  strong  enough  to  support  itself ;  it  may  then  happen 
that  the  lower  fluid  lava  flows  on,  leaving  a  rude  arch  span- 
ning the  cavity  it  occupied.  Buried  beneath  showers  of 
volcanic  dust  and,  perhaps,  overflov/ed  by  lava,  these  cham- 
bers may  become  converted  into  water-reservoirs.  When  the 
water-filled  caverns  are  penetrated  by  the  dykes,  or  fissures, 
filled  with  molten   rock,  the  water  is   suddenly  converted  into 


I40  ASPECTS   OF  THE  EARTH. 

Steam.      In  this  way  such  small  and  temporary  craters  as  those 
which  lie  on  the  flanks  of  Mount  yEtna  may  be  formed. 

Besides  these  larger  cavities  formed  in  lava  in  the  ways 
before  described,  there  are  many  smaller  rifts  which  are 
caused  by  the  shrinkage  of  the  lava  in  cooling.  This  shrink- 
age often  amounts  to  as  much  as  one-tenth  of  the  mass,  and 
leads  to  the  production  of  various  irregular  cavities. 

We  have  now  briefly  considered  the  ways  in  which  the 
empty  spaces  of  the  earth's  crust  are  formed.  We  see  that 
by  several  different  causes  numerous  cavities  come  to  exist. 
It  must  be  observed  that  these  cavities  are  essentially  super- 
ficial ;  it  is  certain  that  they  are  limited  to  the  mere  film  on 
the  surface  of  the  globe.  The  reason  why  all  caverns  must 
be  superficial  phenomena  is  very  simple.  As  we  descend  into 
the  earth  the  pressure  due  to  the  overlying  matter  becomes 
constantly  greater,  until  at  a  depth  of,  say,  twenty  miles  the 
weight  of  the  superincumbent  rock  would  cause  every  empty 
space,  however  strong  its  walls,  to  be  crushed  in.  Even  if 
the  rocks  were  very  rigid,  still  the  weight  would  render  cav- 
erns improbable  at  a  depth  of,  at  most,  a  few  score  miles 
below  the  crusts.  The  only  exceptions  to  this  rule  would  be 
where  small  cavities  were  filled  with  water  or  other  fluids 
which  could  not  flow  out  when  subjected  to  pressure,  or  pos- 
sibly where  very  much  heated  gases  pressed,  with  enormous 
energy,  against  the  weight  of  the  superincumbent  rock.  But 
the  vast  areas  of  granite,  marble,  and  other  crystalline  rocks 
which  have  once  been  buried  at  great  depths  beneath  the  sur- 
face show  us,  by  their  compact  structure  and  the  total  absence 
of  caverns,  that  deeper  parts  of  the  earth  are  destitute  of 
vacant  spaces. 


CAVERNS   AND   CAVERN  LIFE.  141 

Many  speculative  minds  have  fancied  that  the  central 
portions  of  the  earth  were  hollow,  and  in  this  imaginary 
realm  have  found  a  larger  field  for  fancy  than  the  real  cav- 
erns afford.  This  notion  is  an  old  one  ;  it  had  a  certain 
currency  in  Germany  more  than  two  hundred  years  ago.  In 
the  early  part  of  this  century  the  speculation  was  renewed  or, 
more  likely,  separately  invented  by  Captain  Symmes,  of  the 
United  States  Army.  Symmes  was  an  original  genius,  with 
a  more  adventurous  spirit  than  most  speculators.  He  not  only 
proved  to  his  own  satisfaction  the  existence  of  this  gigantic 
"hole,"  but  he  endowed  it  with  a  luminous  atmosphere,  the 
glare  of  which,  shining  through  the  entrance-ways  at  the  poles, 
gave  rise  to  the  aurora  borealis.  In  the  true  explorer  spirit 
he  resolved  to  journey  to  this  nether  realm.  With  eminent 
foresight  he  perceived  that,  when  his  ship  turned  round  the 
sharp  angle  which  had  to  be  passed  in  proceeding  from  the 
outer  to  the  inner  sea,  the  sudden  chano-e  of  direction  mig-ht 
snap  the  masts  away  from  their  fastenings.  He  therefore 
planned  a  strong  vessel  whose  spars  might  be  quickly  lowered 
to  the  deck.  He  issued  invitations  to  many  eminent  men  of 
science  to  accompany  him  on  his  journey.  But,  with  greater 
ofood-fortune  than  attends  most  dreamers,  he  died  before  set- 
ting  sail. 

Although  we  must  dismiss  the  notion  of  a  central  space, 
the  earth  constantly  contains  in  its  more  superficial  parts  a 
great  number  of  cavities,  which  have  an  important  influence 
on  the  deposition  of  minerals  of  value  to  man,  and  which 
afford  a  field  for  the  development  of  a  singular  group  of 
organic  beings.  These  caverns  are  constantly  forming  and 
constantly  being  destroyed.  None  of  the  superficial,  or  cold- 
water,  caves  are   more  than  two  or  three  geological  periods 


142  ASPECTS   OF  THE  EARTH. 

old  ;  they  constantly  vanish  as  the  surface  of  the  earth  wears 
down  to  them.  But  those  of  the  deeper  earth,  formed  by 
the  migrations  of  the  heated  waters,  are  among  the  older 
products  of  water-action  ;  they  may  have  kept  their  forms 
since  a  time  when  the  hills  which  overlie  them  had  not 
begun  to  be  carved  out  by  the  superficial  streams. 


RIVERS  AND  VALLEYS. 


Advantafres  of  Beginning  Study  of  Geology  with  River  Action. — Description  of  a  River 
Valley.  —  History  of  Rain-Drops  ;  Mountain-Torrents  ;  I'rocesses  of  Erosion. — Passage 
from  Torrents  to  Rivers. — Alluvial  Terraces. — Effect  of  Plants  on  Alluvium.— Effect  of 
Tributary  Streams. — Ox-bows  and  Moats. — Function  of  Alluvial  Plains. — Effect  of 
these  Deposits  on  Conditions  of  Ocean  ;  Erosion  of  River  Channels. — Waterfalls. — Classi- 
fication of  Cataracts. — Niagara  Falls  :  Effect  of  their  Recession. — Effect  of  Elevation 
of  Continents. — Base  Level  of  Erosion. — Action  of  Subterranean  Water. — Wanderings  of 
Rivers. — Distribution  of  Streams.  —  Effect  of  Changes  of  Elevation  of  the  Land  on 
Rivers. — Effect  of  Mountain  Systems. — Geological  Consequences  of  Distribution  of 
Rivers. — Comparison  of  Ohio  with  Colorado  River. — Formation  of  Biittes. — Deltas: 
their  Influence  on  Man  ;  Advantages  for  Primitive  Peoples. — Change  and  Destruction 
of  River  Valleys. — Dislocations  ;  Glacial  Deposits  ;  Lava  Streams. — Evidence  from 
Old  River  Channels  Concerning  Antiquity  of  Men. — Effect  of  Forests  on  Rivers. — 
Problem  of  the  Mississippi  River. — Control  of  Floods — Danger  from  Reservoirs  in  River 
Valleys.  —  I  rrigation . 

The  greater  part  of  the  facts  with  which  geologists  have 
to  deal  possess  for  the  general  public  a  recondite  character. 
They  concern  things  which  are  not  within  the  limits  of  famil- 
iar experience.  In  treating  of  them,  the  science  uses  a  lan- 
guage of  its  own,  an  argot  as  special  as  that  of  the  anatomist 
or  the  metaphysician.  There  is,  however,  one  branch  of  the 
subject  the  matter  of  which  demands  no  special  knowledge 
for  its  understanding,  viz.  :  the  surface  of  the  earth.  At  first, 
geologists  were  little  inclined  to  deal  with  the-  part  of  their 
field  which  is  visited  by  the  sun.  Gradually,  however,  they 
have  come  to  see  that  this  outer  face  of  the  earth  is  not  only 
a  kindlier  but  a  more  legible  part  of  the  great  stone  book,  and 
they  have   made   a   division  of  their  work  which  they  entitle 


144  ASPECTS   OF  THE  EARTH. 

Surface  Geology.  In  this  division  they  include  all  that  is 
evident  to  the  untrained  understanding,  the  contour  of  land 
and  of  sea-floor,  the  aspects  of  shores,  the  conditions  of  soil, 
etc.  Under  the  head  of  Rivers  and  Valleys  we  propose  to 
consider  one  portion  of  this  simple  but  ample  division  of 
geologic  science. 

If  the  reader  wishes  to  begin  a  series  of  studies  of  an 
unprofessional  character  which  will  lead  him  to  some  of  the 
most  important  fields  of  knowledge  which  the  earth's  science 
can  open  to  him,  he  cannot  do  better  than  find  his  way.  to  his 
subject  through  a  river-valley.  There  are  many  advantages 
offered  to  him  in  beginning  his  inquiries  in  this  pleasant  way. 
In  the  first  place,  the  outward  aspect  of  the  phenomena  with 
which  he  has  to  deal  is  already  familiar  to  him.  We  can  all 
recall  to  mind  some  of  these  troughs  of  the  earth  through 
which  flows  a  stream,  be  it  mountain-torrent,  brook,  or  river. 
The  steep  or  gentle  slopes  of  the  valley  toward  the  agent 
which  has  constructed  it,  the  flowing  water,  as  well  as  many 
of  the  important  actions  of  the  stream  in  its  times  of  flood 
or  in  its  cataracts,  are  also  familiar.  In  fact,  there  is  not 
a  feature  or  a  phenomenon  visible  in  the  valley  which  has 
not  a  popular  name,  indicating  that  it  is  a  matter  of  com- 
mon and  easy  observation.  Whoever  will  follow  an  ordinary 
stream  from  its  sources  to  the  sea  in  such  a  journey  as  he  may 
make  in  a  few  days'  travelling,  and  will  avail  himself  of  its 
teachings,  with  the  aid  of  the  simplest  understandings  derived 
from  a  knowledge  of  physical  laws,  will  obtain  a  clew  to  a  very 
large  part  of  the  earth's  machinery. 

To  see  the  actual  beginning  of  the  river  under  the  condi- 
tions which  are  best  for  our  inquiry,  we  must  observe  the  sur- 
face at  some  point  on  the  dividing  line  between  two  streams 


RIVERS   AND    VALLEYS.  145 

whe^re  they  head  together,  near  the  crest  of  a  mountain,  in  a 
time  of  rain.  All  that  is  visible  are  the  drops  of  rain  which 
slip  out  of  the  air  and  patter  on  the  surface  of  the  earth. 
We  must  be  prepared  at  the  outset  to  look  past  this  simple 
fact  of  rainfall  and  to  conceive  the  physical  history  of  the 
drop  of  water  since  it  left  the  surface  of  the  earth  in  its 
journey  through  the  clouds  and  back  to  earth  again. 

The  story  of  the  rain-drop  before  it  comes  to  the  earth  is 
very  simple.  The  heat  from  the  sun,  aided  in  a  small  meas- 
ure by  the  heat  from  all  the  stars,  evaporates  the  water  from 
the  earth's  surface,  mainly  from  the  sea,  and  removes  it  in  the 
state  of  vapor  to  a  height  of  many  thousand  feet  above  the 
earth's  surface.  It  is  maintained  there  by  the  heat  which  it 
has  absorbed,  and  thus  the  main  spring  of  the  rain  is  in  the 
sun.  After  abiding  awhile  in  the  upper  regions  of  the  atmos- 
phere, by  some  of  the  many  chances  which  beset  the  clouds, 
the  vapor  is  cooled  ;  it  condenses  from  the  loss  of  heat,  and 
falls  as  rain  or  snow.  The  circumstances  of  our  imaginary 
mountain  top,  if  that  summit  be  at  a  considerable  height 
above  the  sea,  favor  the  coolino-  of  the  cloud  and  therefore 
the  precipitation  of  this  rain.  These  uplands  retain  the  cold 
of  winter,  and  during  night  they  pour  forth  their  heat  by 
radiation  through  the  thin  air,  with  more  rapidity  than  the 
lower  lands,  which  are  covered  beneath  a  thicker  blanket  of 
atmosphere. 

When  the  drop  of  rain  falls  to  the  earth's  surface,  if  it  be 
of  ordinary  size,  it  gives  a  sensible  blow.  If  that  surface  be 
covered  with  a  thin  layer  of  scattered  sand-grains  or  small 
pebbles,  we  may  observe  that  the  bits  of  rock  dance  about 
and  thus  apply  a  little  of  the  force  which  comes  from  the 
drop,  to   rub  the  stone  on  which   they  lie.      At  first,  the  water 


146  ASPECTS    OF  THE  EARTH. 

spreads  over  the  earth's  surface  as  a  thin  sheet,  but  as  that 
surface  is  never  perfectly  level,  it  is,  provided  the  rock  be 
bare,  quickly  gathered  Into  rivulets;  or  if  it  be  covered  with 
mosses,  or  the  thin  layer  of  porous  soil  common  to  mountain- 
tops,  it  may  for  a  moment  disappear  from  sight  in  the  spongy 
mass  ;  but  a  little  farther  down,  we  find  that  it  is  gathered  in 
rivulets,  which  quickly  join  together,  so  that  in  descending 
even  a  hundred  feet  below  the  summit,  in  a  time  of  rain,  we 
find  a  number  of  shallow  valleys,  each  occupied  by  a  little 
rivulet.  The  union  of  these  streams  gives  us  one  of  more 
power,  which  may  be  taken  as  a  typical  mountain-torrent. 
We  observe  that  such  a  stream  descends  with  considerable 
rapidity  ;  it  is  rare  indeed  that  it  does  not  have  a  fall  of  more 
than  fifty  feet  to  a  mile.  The  rate  of  fall  in  steep-faced 
mountains  often  amounts  to  as  much  as  five  hundred  feet  in 
that  distance.  As  soon  as  the  stream  is  more  than  two  or 
three  feet  wide  and  a  foot  in  depth,  we  begin  to  see  evidences 
of  its  energy.  Even  if  the  fall  be  but  at  the  rate  of  fifty  feet 
to  the  mile,  we  shall  find  that  such  a  stream  is  able  to  urge 
forward  with  great  violence  masses  of  stone  several  inches  in 
diameter.  If  we  roll  a  stone  the  size  of  a  man's  head  into 
the  channel,  it  is  swept  along,  bumping  violently  against  the 
obstacles  it  encounters,  striking  first  one  rock-bank  and  then 
another,  until  it  becomes  fixed  in  some  crevice.  If,  after  the 
pebble  has  journeyed  for  a  few  hundred  feet,  we  recover  it 
from  the  stream,  it  is  often  easy  to  note  the  dents  on  its 
surface,  produced  by  the  collisions  on  its  journey.  In  niost 
cases  there  has  been  a  corresponding  blow  and  an  equal 
wearing  inflicted  on  the  firm  rocks  arainst  which  it  collided. 

A  little  observation  with  streams  having  different  rates  of 
fall   will   show  the  observer  that  the  ease  with  which  a  stone 


RIJ'ERS  AXD    J 'ALL  ATS. 


H7 


is  urged  onward,  and  the  size  of  those  which  a  stream  of 
given  volume  can  carry,  depend  in  a  remarkable  way  on  the 
rate  of  its  descent  toward  the  sea  level,  and  therefore  on  the 
velocity  with  which  its  waters  flow.  Computation  and  experi- 
ence have  shown  that  this  increase  in  speed  is  proportionate 
at  least  to  the  cube,  or  third  power,  of  the  velocity  with  which 
the  current  flows.      One  distinguished   student  of  this  hydrau- 


Torrent  Bed  in  Eastern  Kentucky. 
(Showing  channel  embarrassed  by  masses  of  stone  fallen  from  the  sides  of  the  valley.) 

lie  problem  has  come  to  the  conclusion  that  the  increase 
of  the  propulsive  power  of  the  stream  upon  the  fragments 
which  it  encounters  is  as  the  sixth  power  of  its  speed.  It  is 
not  worth  while  for  us  to  pause  in  our  imaginary  journey  to 
consider  whether  the  third  power  or  the  sixth  be  the  rate 
at  which  the  efficiency  in  the  carrying  power  of  the  stream 
increases  with  its  speedier  flow.  It  is  enough  for  us  to  know 
that  the  water,  with   very  slight   increase  in    its   velocity,    is 


14S  ASPECTS    OF  THE  EARTH. 

able  to  carry  a  very  much  larorer  stone  than    It  could   before 
its  speed  was  increased. 

The  sides  of  these  mountain-torrents  are  generally  steep. 
It  is  rare  indeed  that  the  slopes  which  lead  to  them  are  much 
less  inclined  than  the  roofs  of  ordinary  houses.  Over  all  the 
surface  on  either  side  of  the  torrent,  frost  and  other  agents 
of  decay  are  constantly  at  work  breakinor  out  bits  of  stone 
or  forming  soil.  This  mass  of  broken-up  rock  is  constantly 
slipping  down  the  sides  of  the  yalley.  Eyery  time  the  winter 
frost  seizes  it.  it  expands  a  little,  and  is  thus  shoyed  down- 
ward ;  frequently,  when  soaked  with  water,  great  sheets  of  it 
slip  swiftly,  as  mud-ayalanches.  into  the  stream.  In  this  way 
the  torrent  is  always  proyided  with  fragments  which  it  may 
o-rind  up  into  pebbles,  sand,  and  mud,  and  bear  onward  to 
the  fields  below.  In  times  of  drought,  these  stream-beds  are 
occupied  by  rivulets  of  clear  water,  and  at  such  periods  the 
observer  eains  no  idea  of  the  yiLJor  with  which  the  niill  works; 
but  in  times  of  heavy  rain  he  will  find  the  water  turbid  with 
sediment  made  b\-  the  attrition  of  pebbles  against  the  border- 
ing walls  of  the  stream  and  upon  each  other.  He  then  sees 
whence  come  the  sediments  which  are  so  important  a  feature 
in  the  lower  portions  of  the  river-system.  From  any  com- 
manding elevation  in  a  mountain  district,  we  may  see  scores 
or  hundreds  of  those  torrent-beds  within  one  field  of  view. 
In  periods  of  heav)-  rain,  the  roar  arising  from  the  moving 
stones  is  ofttm  a  very  striking  feature. 

Descending  the  channel  of  any  of  these  mountain  torrents, 
we  find  that  after  a  few  miles  of  course,  though  the  brook 
steadily  gains  in  volume  by  the  contributions  of  tributary 
streams,  it  gradually  diminishes  the  swiftness  of  its  descent. 
At  a  certain   point   it  ceases  to  bear  onward  all  of  the  larger 


RIVERS   AXB    VALLETS. 


149 


Stones  which  come  Into  its  possession.  These  fragments 
gather  upon  the  banks,  forming  a  rude  terrace.  Still  farther 
down,  where  the  slope  is  less  considerable,  the  smaller  pebbles 
are  left  behind,  crowded  into  the  interstices  of  the  laro-er  frao-- 
ments.    The  terrace  becomes  more  distinct,  ves^etation  leathers 


II  f  "Bijifc  oir'an"% 


Upon  it,  and  the  waste  01 
the  plants  forms  a  soil 
which  partially  levels  off 
the  surface.  Farther  on. 
we  come  to  the  field  where 
the  annual  overflow  of  the 
stream  during  the    spring 

floods  heaps  a  quantity  of  the  sand  and  mud  upon  this  foun- 
dation of  coarser  material  ;  we  then  have  the  beorinninof  of  the 
alluvial  terrace.  At  first  this  alluvial  terrace  is  but  a  nar- 
row belt  on   either    side    of  the   stream,  which,  swollen  bv  its 


SnoA.ng  ;.'■€   Beg ■•'■."•■»£  o:  Kcm   Terraces,  ji.~~.   Z; 
the  Torrential  Portion  of  the  Stream. 


I50  ASPECTS    OF   THE  EARTH. 

flood-waters,  often  breaks  new  channels  through  this  bench  of 
detrital  matter.  In  fact,  all  this  marginal  accumulation  is 
of  temporary  duration,  for  the  stream  is  as  yet  wild,  and  in  its 
annual  floods  is  apt  to  undo  the  construction-work  of  the 
previous  years. 

When  the  stream  comes  to  have  a  distinct  and  somewhat 
enduring  alluvial  belt  on  either  side  of  its  path,  it  has  entered 
on  the  stage  of  a  river.  It  is  indeed  on  the  presence  of  this 
marginal  accumulation  that  we  most  rest  the  distinction  be- 
tween a  torrent  and  a  river.  From  the  place  where  the  ter- 
races begin  to  form,  downward  to  the  mouth  of  the  stream, 
the  conditions  of  its  flow  are  vastly  affected  by  its  reactions 
upon  this  detrital  matter.  In  most  cases,  with  each  mile  of 
its  descent  the  magnitude  of  these  deposits  increases.  The 
alluvial  lands  stretch  farther  and  farther  on  either  side  ;  the 
materials  which  compose  them  grow  finer  and  finer  as  we 
descend  in  the  valley,  for  the  reason  that  with  this  descent 
the  slope  of  the  stream  in  most  cases  steadfastly  diminishes 
and  its  ability  to  urge  forward  coarse  sediments  decreases  in 
a  rapid  ratio. 

The  alluvial  deposits  which  border  our  rivers  owe  their 
existence  to  the  fact  that  the  torrential  head-waters,  by  their 
great  velocity,  bear  forward,  beyond  the  mountain  districts,  a 
laro-e  amount  of  materials  which  are  of  such  a  coarse  nature 
that  the  larger  but  less  powerful  lower  part  of  the  stream  can- 
not urge  them  onward  to  the  sea.  In  all  its  journey  to  the 
ocean,  the  river  is  continually  struggling  with  this  detritus. 
It  deals  with  this  burden  in  the  following  manner :  The 
motion  of  the  stream  is  swiftest  in  its  central  parts,  because, 
in  most  cases,  the  water  is  deepest  in  that  part  of  its  bed, 
and  is  therefore  the    least    influenced  by  friction.       On    the 


R/VERS   AXD    VALLEYS. 


15' 


sides  of  the  stream  where  the  water  is  shoal,  the  current  is 
least  swift  ;  therefore  in  these  mars^inal  parts  it  constantly 
tends  to  lay  down  sediments.  As  soon  as  the  alluvial  terrace 
is  formed,  certain  kinds  of  trees,  particularly  our  willows  and 
aspens,  find  a  lodgement  upon  it.  They  push  their  roots  out 
into  the  nutritious  mud  and  enmesh   it   in  their  net-work  of 


Border  of  Alluvial  Terrace  on  Green  River,  Ky. 
(Showing  the  manner  in  which  the  forest  occupies  and  protects  the  lower  terrace  of  the  valley.) 

fibres  ;  they  also  send  up  from  these  roots  a  thick  hedge  of 
stems,  in  which  the  flood-waters  lose  their  swiftness  of  motion 
and  therefore  drop  their  contained  sediments.  In  the  state  of 
nature,  all  our  American  streams,  and  those  of  most  other 
countries  as  well,  are  bordered  by  a  close  array  of  these 
plants,  all  of  which  are  at  work  to  win  against  the  channel 
of  the  stream.  But  for  the  cutting  power  of  the  stream, 
they  would  quickly  close  its  channel  ;  as  it  is,  they  constantly 
crowd  its  waters  within  a  narrow  pathway. 


152  ASPECTS    OF   THE  EARTH. 

Against  the  encroachments  of  the  alluvial  banks  brought 
about  by  the  action  of  the  water-loving  trees,  the  river  pre- 
vails by  fits  and  starts,  under  the  action  of  a  curious  law 
which  causes  its  current  to  rebound  from  bank  to  bank.  The 
nature  of  this  principle  of  rebounding  can  best  be  seen  by 
observing  the  effect  arising  where  a  jetty  is  built  at  any  point 
in  the  course  of  one  of  our  larger  rivers.  The  jetty  causes 
the  water  to  sweep  away  from  its  obstruction  and  to  strike 
against  the  opposite  shore.  The  crowding  against  the  shore 
gives  its  current  increased  power;  it  will  wrest  away  the  allu- 
vium from  the  grasp  of  the  roots,  and  will  then  cut  under  the 
trees,  causing  considerable  areas  of  forests  to  be  precipitated 
into  the  waters  and  borne  away  to  the  sea.  From  the  point 
of  impact,  the  current  will  again  rebound  in  a  manner  which 
will  cause  it,  at  a  certain  distance  below,  to  strike  against  the 
opposite  bank,  where  it  will  again  make  swift  encroachment 
against  the  forest-protection.  After  this  second  assault,  it 
will  swing  across  to  a  lower  point  on  the  shore  against  which 
it  first  impinged,  and  so  the  oscillations  from  side  to  side  will 
be  propagated  down  stream,  it  may  be  for  a  hundred  miles 
or  more.  A  single  jetty  of  this  description,  as  it  has  been 
observed  in  the  rivers  of  India,  will  affect  the  oscillations  of 
the  current  for  an  indefinite  distance  downward  in  its  course. 
That  which  is  accomplished  by  artifice  in  an  immediate  man- 
ner is  more  slowly  brought  about  by  natural  causes.  Each 
tributary  stream  which  enters  the  main  channel  commonly  has 
a  greater  swiftness  of  current  than  the  larger  stream  into 
which  it  flows.  It  therefore  bears  in  a  mass  of  pebbles  and 
builds  a  natural  jetty  or  bar  at  its  mouth,  thus  gradually  forc- 
ing the  current  of  the  larger  stream  against  the  opposite  side, 
creating  a   bar  there.      It   is   furthermore   to    be    noted,  as   is 


RIVERS  AND    VALLEl'S. 


00 


shown  in  the  diagram  (p.  154),  that  between  the  points  where 
the  river  impinges  against  the  bank  there  is  a  space  of  dead 
water  or  eddying  currents  in  which  the  forests  find  it  easy  to 
make  head  against  the  river  and  to  extend  the  alluvial  plain. 


Cumberland  River,   Ky.,  from  Taylor's  Hill, 

(Showing  the  relation  of  alluvial  plains  on  upper  portion  of  the  river  to  the  hills  which  form  the  valley  ; 
also  the  beginning  of  the  true  river-curves  formed  by  the  struggle  of  the  stream  with  its  sediments. 
Photo,  by  Ky.  Geol.  Survey.) 

Thus,  in  the  process  of  nature,  it  comes  about  that  our 
rivers  tend  to  build  channels  in  their  alluvial  plains  which  are 
extremely  devious  in  their  course.  If  the  alluvial  plains  be 
wide,  the  river  is  constantly  forming  great  ox-bow-like  curves, 
isthmuses  with  narrow  peninsulas  such  as  are  often  seen  in  the 
lower  portions  of  the  Mississippi  valley.  Finally  the  narrow 
places  which  connected  these  promontories  on  the  shore  are 
cut  through  in  some  time  of  flood,  the  river  finding  a  shorter 
way  downward  to  the  sea,  leaving  its  former  circuit  as  a  great 


154  ASPECTS    OF  THE   EARTH. 

pool,  or  moat,  as  it  is  called  by  the  common  folk  along  the 
banks  of  the  Connecticut  Riven*  It  often  happens  in  the  low- 
er Mississippi  that  the  course  of  the  river  around  the  prom- 
ontory of  the  ox-bow  is  ten  or  more  miles  in  length,  while  the 
space  across  the  neck  is  less  than  a  mile  in  distance.  When 
the  river  finally  breaks  across  the  neck,  the  whole  system  of 
rebounds  of  its  current  against  the  banks,  from  the  point  of 
change  downward  to  the  mouth,  may  become  altered.  The 
points  which  before  were  in  process  of  erosion   may  become 


Diagram  Showing  the  Wanderings  of  a  Stream  in  an  Alluvial  Plain. 

( The  arrows  on  the  sides  of  the  stream  indicate  the  direction  of  its  movement  ;  the  horseshoe-shaped 
pool  is  an  "  ox-bow  "  or  "  moat  "i 

the  seats  of  deposition,  and  those  which  previously  were  gain- 
ing may  begin  to  wear  away.  In  this  manner  a  river,  in  time, 
wanders    to  and   fro  across  its  whole  valley,   taking  material 

"This  term  "moat"  deserves  a  place  in  our  geological  language,  for  the  reason 
that  it  is  a  brief  and  expressive  word  for  the  topographic  feature,  ill-described  in  our 
present  system  of  naming.  Moreover,  it  preserves,  in  an  interesting  way,  a  memory 
of  mediaeval  conditions.  The  name  was  doubtless  given  because  of  the  likeness 
which  the  early  settlers  saw  between  these  circular  ditch-like  pools  and  the  defences 
which,  in  the  seventeenth  century,  were  still  familiar  objects  about  many  oi  the 
country  houses  in  Great  Britain.  I  shall  therefore  use  the  term  in  the  jiresent  writ- 
ine  and  hereafter  in  the  sense  above  indicated. 


RIVERS   AXD    VALLEYS.  1 55 

from  one  side,  sorting  it  over,  removing  that  part  which  is  fine 
enough  to  be  borne  away  by  the  current,  and  rebuilding  the 
remainder  into  the  alluvial  plains. 

We  are  now  prepared  to  consider  a  very  peculiar  and  most 
important  function  which  these  alluvial  plains  perform  in  the 
physical  life  of  the  earth.  In  such  a  valley  as  the  Mississippi, 
we  have  probably  not  less  than  fifty  thousand  square  miles  of 
alluvial  plains  which  have  been  formed  of  the  waste  removed 
from  the  rocks  in  the  torrential  portions  of  the  streams  in  the 
mountain  and  hill  districts  of  the  valley.  This  alluvial  mate- 
rial is,  on  the  average,  not  less  than  fifty  feet  thick.  It  is 
therefore  equivalent  to  about  five  hundred  cubic  miles  of  mat- 
ter. Now,  this  great  river  carries  out  to  sea  about  one-twen- 
tieth of  a  cubic  mile  of  sediment  each  year.  This  sediment 
which  goes  into  the  sea  is  in  small  part  directly  derived  from 
the  action  of  the  mountain-torrents  ;  in  larger  part,  it  is  com- 
posed of  waste  taken  from  the  alluvial  plains  by  the  wander- 
ings of  the  various  streams  which  constitute  the  Mississippi 
system  of  waters.  It  therefore  follows  that  the  average  time 
required  for  the  sediment  discharged  from  the  mouth  of  the 
Mississippi  to  make  its  way  from  the  head-waters  to  the  sea 
is  not  less  than  ten  thousand  years.  As  soon  as  a  pebble  or 
other  bit  of  rock  is  laid  away  in  the  alluvial  terrace,  it  begins 
to  decay  ;  the  vegetable  acids  which  penetrate  the  mass  in 
which  it  finds  lodgement  favor  its  disintegration.  W^hen  it  is 
turned  over  by  the  stream  at  the  time  of  encroachment  on  its 
resting-place,  it  probably  falls  to  pieces,  the  finer  bits  are  hur- 
ried onward  by  the  stream,  those  too  coarse  for  the  current  to 
control  are  again  stored  away  in  the  bank  to  await  further 
decay.  In  this  manner  the  alluvial  material  lying  on  either 
side  of  rivers   is  a  great  storehouse,  or  rather  we  should  say 


156  ASPECTS    OF  THE  EARTH. 

laboratory,  in  which  sediments  are  divided  and  brought  into 
a  chemical  condition  which  permits  them  to  be  taken  into 
the  control  of  the  waters  and  borne  away  to  the  ocean,  in 
order  to  become  rebuilt  into  strata,  which  are  in  time,  with 
the  growth  of  the  continents,  to  become  dry  land  and  be 
again  subjected  to  this  erosive  work.  Were  it  not  for  this 
system  of  alluvial  storage  and  decay,  the  seas  could  not  be 
supplied  with  the  debris  essential  for  the  maintenance  of  the 
life  which  they  contain  ;  for  that  life,  unlike  the  life  of  the 
land,  does  not  depend  on  the  soil  of  the  ocean  floors,  but 
upon  the  dissolved  matter  contained  in  the  water,  from 
which  the  marine  animals  and  plants  take  all  their  store  of 
nutrition.  This  nutrition  comes  mainly  from  the  land-waste 
brought  to  the  sea  in  the  state  of  solution  bv  the  streams, 
and,  as  we  have  just  seen,  the  comminution  and  solution  of 
this  waste  depend  upon  the  work  which  goes  on  in  the 
laboratories  of  the  alluvial  plains. 

It  is  true  that  a  portion  of  the  mineral  matter  contributed 
by  the  land  to  the  sea  comes  from  the  seashore,  and  yet 
another  portion  from  volcanic  ejections  which  are  poured  out 
from  the  numerous  vents  of  oceanic  islands.  The  material 
taken  from  the  seashore  into  solution  by  the  sea  water  is, 
however,  small  in  quantity,  and  this  for  the  reason  that  the 
ocean  water  has  usually  but  a  small  amount  of  free  carbonic 
acid  to  aid  in  its  solvent  work.  The  material  contributed 
from  volcanoes  is  larger  in  quantity  than  that  won  by  the 
ocean  waves  from  the  coast  line.  A  large  part  of  this  vol- 
canic waste  is,  however,  borne  to  the  ocean  from  the  land  on 
which  it  falls,  by  the  streams,  which,  as  explained  in  the  chap- 
ter on  Volcanoes,  readily  remove  the  incoherent  volcanic 
waste  by  the  action  of  their  waters,  and  bear  it  to  the  sea. 


RIVERS   AND    VALLEYS. 


157 


We  have  now  seen  the  way  in  which  the  water  operates 
upon  the  surface  of  the  stream-beds.  At  the  source  of  the 
mountain-torrents,  a  pound  of  water  has  in  it,  by  virtue  of 
its  height  above  the  level  of  the  sea,  a  great  store  of  energy, 
which  it  may  apply  to  the  erosion  of  the  earth's  surface.  Let 
us  suppose  that  when  it  comes  to  the  earth  it  is  three  thou- 
sand feet  above  the  ocean's  level.      It  has  then  as  much  force 


Showing  Alluvial  Terraces  of  Soft   Material  Rapidly  Eroded  hy   a    River,  wfiich    is   Constructing  what  in  Titn& 

will  be  a  yet  Lower  Terrace. 

to  expend  as  would  be  required  to  lift  it  to  that  height  above 
the  sea.  At  first  the  stream  plays  the  part  of  spendthrift 
with  this  energy;  the  greater  portion  of  the  force  is  expended 
in  brawling  with  the  stones  and  in  beating  against  the  limits 
which  confine  it.  In  the  first  five  miles  or  so  of  its  path  to 
the  sea  it  uses  up  in    its    descent    perhaps    one-third    of    its 


158  ASPECTS    OF  THE  EARTH.    . 

dynamic  resources,  and  so,  for  the  last  thousand  miles,  it 
may  not  have  more  power  at  its  command  than  it  gave  out 
in  the  first  five  miles  of  its  journey. 

Thus  our  streams,  though  always  growing  larger,  are  con- 
tinually becoming  less  and  less  powerful  in  proportion  to  the 
weight  of  water  which  flows  over  their  beds.  In  the  lower 
portion  of  their  courses  they  have  very  little  capacity  for 
eroding  the  rocks  over  which  they  fiow,  except  where  that 
yjower  is  due  to  some  peculiar  circumstances.  They  deepen 
their  beds  slowly,  and  the  greater  portion  of  this  deepening  is 
accomplished  by  the  corrosion  or  chemical  decay  of  the  rocks 
over  which  they  flow.  Still,  certain  peculiar  circumstances 
may  give  them  a  chance  to  cut  down  the  floors  of  their  lower 
channels.  This  work  is  done  in  either  of  the  following  ways  : 
When  the  lateral  swina-ing;  of  the  river-beds  to  and  fro  throua;h 
the  alluvial  plain  dislodges  great  forest-trees  from  the  bank, 
these  trees  often  have  great  quantities  of  stones  entangled  in 
their  roots.  These  roots  are  thus  held  against  the  bottom 
while  the  trees  are  swept  onward  by  the  current,  and  so  the 
entangled  stones  rasp  upon  the  bed  and  serve  to  wear  the 
channel  deeper.  Again,  it  often  happens  in  cold  countries 
that  the  rivers  are  deeply  frozen,  and  during  the  winter  sea- 
son, in  the  shallow  water,  the  loosened  stones  of  the  bottom 
may  be  entangled  in  the  ice.  When  the  time  of  "  break- 
ing up  "  comes,  the  sheets  of  ice,  as  they  float  downward  in 
oreat  fields,  strike  against  the  banks  of  the  river  where  there 
is  a  sharp  bend  in  the  channel,  and,  owing  to  their  great 
momentum,  are  heaped  up  in  a  wall  of  fragments,  which 
may  in  a  few'  minutes  dam  the  river  quite  across.  Owing 
to  the  pressure  to  which  these  cakes  of  ice  are  subjected, 
they  freeze  together,  and  the  whole  of  one  of  these  ice  dams 


Stream  Bed  with  Bowlders  Formea  from  Angular  Masses  Rolled  in  Times  of  Flooo 


RIVERS  AND    VALLEYS.  159 

or  gorges  becomes  a  solid  mass.  When  this  happens,  as  is 
easily  conceived,  the  stream  rises  rapidly,  forming  a  great 
lake  above  the  dam,  while  it  drains  away  below,  and  thus, 
as  in  the  Ohio  River,  these  dams  may  have  a  difference  of 
twenty  or  thirty  feet  of  water  above  and  below  their  obstruc- 
tions. In  a  brief  time  the  pressure  of  the  water  above  the 
dam  pushes  the  whole  mass  forward,  grinding  it  upon  the 
bottom  and  the  sides,  and  so  powerfully  eroding  the  rock- 
bed  in  which  the  stream  flows. 

As  long:  as  the  river  flows  onward  over  rocks  of  uniform 
hardness,  especially  where  the  strata  lie  in  horizontal  attitudes, 
the  course  of  the  stream  generally  exhibits  a  uniform  descent. 
Various  accidents  in  the  attitude  of  the  rocks  may,  however, 
give  rise  to  rapids  or  waterfalls.  These  features  in  the  course 
of  a  river  are  so  important  in  its  mechanism,  especially  with 
reference  to  the  interests  of  man,  that  they  deserve  a  careful 
consideration,  which  we  shall  now  give  to  them. 

Waterfalls  and  rapids  owe  their  existence  in  the  main  to 
one  of  three  conditions  of  the  bed-rock.  These  conditions 
are  as  follows:  First,  the  path  of  the  stream  may  be  crossed 
by  a  dike  or  a  vein,  which  are  rifts  in  the  rocks,  filled  with 
some  deposit  brought  into  them  by  the  action  of  water  or 
forced  to  its  place  in  the  condition  of  a  lava.  Where  these 
dike-  or  vein-materials  are  softer  than  the  neighboring  rock 
over  which  the  stream  flows,  the  river  easily  cuts  them  down 
and  they  create  no  interruption  to  its  course.  Where,  how- 
ever, as  is  often  the  case,  the  rocks  which  fill  the  fissures 
are  harder  than  the  materials  which  formed  its  walls,  the  river 
is  obstructed,  and  we  generally  have  a  cataract,  that  is,  an 
irregular  fall,  in  which  the  stream  takes  no  one  conspicuous 
plunge.      Another  case   in  which    a    local    hardening    of    the 


l6o  ASPECTS  OF  THE  EARTH. 

stream-bed  produces  a  waterfall  is  where  a  stream,  flowing 
over  rocks  which  may  be  horizontal  in  their  attitude,  encoun- 
ters a  coral  reef,  formed  on  the  old  sea-floors  in  which  the 
strata  were  deposited.  In  this  case  the  crowding  together  of 
the  fossil  corals  may  make  the  rock  much  firmer  than  the 
neighboring  portions  of  the  strata,  and  so  produce  a  decided 
interruption  in  the  uniform  descent  of  the  stream.  Only  one 
important  case  of  reef-cataract  is  known  to  me, — that  which 
occurs  in  the  Ohio  at  Louisville,  where  coral-reef  in  the 
Devonian  period  has  so  far  interrupted  a  gentle  descent  of 
the  river  as  to  create  a  formidable  obstruction,  only  passable, 
save  during  the  flood-times  of  the  river,  by  means  of  a  canal 
extending  from  the  head  to  the  base  of  the  rapid.  The  most 
common  condition  which  leads  to  the  formation  of  a  water- 
fall, the  condition  which  gives  us  the  greater  part  of  the  fine 
falls  of  the  world,  is  where  a  river  flows  across  strata  which 
dip  or  sink  downward  in  the  earth  toward  the  head-waters  of 
the  stream.  In  this  condition,  wherever  a  hard  bed  of  the 
strata  overlies  a  soft  deposit,  the  stream  inevitably  forms  a 
waterfall. 

The  first  two  of  the  above-named  classes  of  waterfalls 
demand  no  very  extensive  consideration.  Those  produced 
by  dikes  and  veins  are  generally  conspicuous  only  in  the 
torrential  portion  of  a  river-system.  The  veins  and  dikes 
account  for  a  very  large  part  of  the  little  cataracts  which  di- 
versify our  mountain-torrents.  Coral-reefs  are  so  rare  in  our 
older  rocks  that  they  are  seldom  cut  by  the  streams,  and  are 
therefore  not  often  seen,  even  by  the  professional  student  of 
geology.  The  third  group,  in  which  each  plunge  of  the  fall 
is  due  to  the  upstream  slope  of  strata,  alone  demands  some 
special  consideration. 


RIVERS  AXD    VALLEYS.  l6l 

Falls  due  to  inclined  strata  can  best  be  represented  by 
Niagara,  perhaps  the  noblest  of  all  such  geological  accidents. 
As  is  shown  in  the  diagram,  we  have  at  Niagara  Falls  a  toler- 
ably hard  layer  of  limestone,  belonging  to  a  division  of  the 
Silurian  age,  which  has  indirectly  received  its  name  from 
this  great  cataract.  This  Niagara  limestone  is  underlaid  by 
a  considerable  thickness  of  softer  shaly  rocks  known  as  the 
Clinton  group.  The  waters  of  the  Niagara  River  plunge 
over  the  hard  rim  afforded  by  the  limestone  and  descend 
about  a  hundred  and  seventy  feet,  acquiring  in  this  movement 
a  very  great  velocity.  At  the  base  of  the  fall,  the  water 
strikes  against  a  mass  of  hard  fragments  which  in  succession 
have  tumbled  down  from  the  resisting  upper  layer.  These 
fragments,  set  violently  in  motion,  cut  out  the  soft  material, 
the  erosion  of  which  is  also  aided  by  the  violent  whirls  of 
water  and  of  spray  driven  against  the  shaly  beds  in  the  space 
behind  the  fall.  From  this  wearing  action,  the  soft  materi- 
als are  constantly  working  backward  more  rapidly  than  the 
hard  upper  layer  is  worn  away,  and  so,  from  time  to  time,  the 
projecting  shelf  over  the  waterfall  is  deprived  of  support  and 
tumbles  to  the  base  in  fragments,  which,  in  turn,  are  used  for 
the  further  erosion  of  the  soft  deposits.  In  Niagara,  as  in 
all  other  waterfalls  of  this  description,  the  border  of  rock 
over  which  the  plunge  takes  place  is  constantly  and  pretty 
rapidly  working  up  stream.  The  fall  Is  progressively  decreas- 
ing in  height,  as  is  shown  in  the  ficjure  ;  and  in  the  end, 
when  the  hard  layer  has  descended  to  the  general  level  of  the 
stream-bed,  especially  when  the  softened  limestone  rocks  have 
passed  altogether  below  that  level,  the  fall  will  disappear — 
first  passing  into  the  stage  of  a  cataract,  and  afterward  van- 
ishing altogether. 


l62 


ASPECTS    OF  THE  EARTH. 


In  the  case  of  Niagara  Falls  the  rate  of  retreat  is  about 
three  feet  in  a  century.  This  rate  is  very  variable.  It  was 
probably  more  rapid  in  the  past  than  at  present,  for  the 
reason  that  the  undercutting  power  of  the  falling  water  dimin- 
ishes with  the  decrease  in  the  height  of  the  precipice  over 
which  it  plunges,  and  this  height  has  been  growing  less  and 
less  ever  since  the  fall  began  to  be.      Although  the  retreat  of 


Diagram  of  Waterfall  of  Niagara  Type. 

(Observe  the  effect  of  hard  limestone  in  determining  the  position  of  the  top  of  the  fall.     Note  that  as 
this  is  worn  away  the  vertical  plunge  will  be  diminished.) 

the  fall  is  slow,  it  will  in  a  very  brief  time,  in  the  geological 
sense  of  that  word,  lead  to  certain  momentous  consequences. 
When  the  hard  layer  of  Niagara  limestone  passes  below  the 
bed  of  the  river,  the  stream  will  then  cut  upon  rocks  of  an- 
other constitution,  making  for  a  time  certain  small  falls  at  a 
hiofher  eeolo^ical  level ;  but  in  the  course  of  ao;'es,  much  less 
long  than  those  which  have  elapsed  since  the  birth  of  this 
waterfall,  the  gorge  of  the  river  will  extend  up  into  the  basin 
of  Lake   Erie,  draining  away  a  considerable  portion  of  that 


RIVERS  AXD    VALLEYS.  1 63 

fresh-water  sea.  We  shall  then,  if  the  continent  retains  its 
present  height  above  the  level  of  the  sea,  have  another  system 
of  cataracts,  in  the  passage  between  Lake  Erie  and  Lake 
Huron,  which  will  also  in  time  be  worn  away.  Other  cata- 
racts will  then  form  at  the  exit  of  Lake  Michigan  ;  and  thus 
the  lower  lakes  of  our  great  American  system  would  be 
diminished  in  area,  or  perhaps  even  disappear.  At  a  yet 
later  stage,  we  may  look  for  diminution  in  the  size  of  Lake 
Superior,  though  that  basin,  owing  to  the  strong  wall  which 
separates  it  from  the  lower  lakes,  is  destined  to  endure  long 
after  the  last-named  basins  have  been  diminished  or  entirely 
drained  away. 

From  these  considerations  we  perceive  how  important  the 
movement  of  waterfalls  may  be  in  determining  the  water  level 
of  extensive  areas.  Not  only  may  their  retreat  lead  to  the 
drainage  of  extensive  inland  seas,  but  as  they  move  up  stream, 
the  drainage  of  all  the  tributary  rivers,  the  mouths  of  which 
are  in  turn  passed  by,  have  their  systems  of  flow  changed 
in  an  important  manner.  Thus,  when  Lake  Erie  is  drained 
away,  a  number  of  subordinate  waterfalls  will  be  developed 
along  the  streams  which  now  empty  into  that  basin.  Each  of 
these  in  turn  will  take  up  its  march  toward  the  head-waters  of 
the  river  in  which  it  forms  ;  and  so  the  effect  of  the  retreat 
of  one  great  waterfall  may  be  propagated  over  the  whole 
surface  of  the  land  which  is  drained  by  a  great  stream. 

The  effect  of  a  retreating  waterfall  deserves  to  be  consid- 
ered with  some  attention,  for  the  reason  that  it  will  afford  the 
student  the  means  of  understanding  how  far  the  structure  of 
the  rocks  in  a  country  may  influence  the  erosion  which  water 
brings  to  its  surface.  Each  of  these  hard  layers  of  rocks, 
as  well  as  the  other  classes  of  dams  which  create  waterfalls, 


1 64  ASPECTS    OF  THE  EARTH. 

tends,  by  determining  the  rate  of  flow  of  the  streams,  to  fix 
the  rate  of  erosion  in  all  parts  of  the  river-basin  above  the 
point  where  they  occur.      Whenever  such  obstructions  are  cut 


Cascaria  <!••    a  ',v!-r;i     II. -ir  La  (ju,.y'a,   Venezuela, 
(Showing  stream  divided  by  following  joint  planes.) 

away,  they  increase  the  rate  of  fall  in  the  waters  above  them  ; 
and  so  this  may  greatly  enhance  the  rate  of  down-wearing  of 
the   surface.      The  erosive  action   of   the  water  which   passes 


RIVERS   AXD    VALLEYS.  1 65 

out  of  a  river  is  determined  by  the  height  through  which  this 
water  descends  in  every  part  of  its  course.  Whatever  tends 
to  increase  the  speed  of  fall  in  the  particular  portion  of  the 
basin  serves  to  magnify  the  erosive  work  in  that  region. 
Thus,  when  a  fall  disappears,  the  energy  which  was  ineffect- 
ively applied  at  the  base  of  its  cliff  may  become  distributed 
over  a  wide  surface  in  the  upper  portion  of  the  valley  in  which 
it  lay. 

So,  too,  in  a  larger  way,  as  the  continents  sink  down  into 
or  rise  above  the  level  of  the  sea,  in  their  ceaseless  oscilla- 
tions, each  movement  is  attended  by  a  great  variation  in  the 
energy  with  which  the  streams  act  upon  their  surface.  If  our 
continent  should  rise  a  hundred  feet  in  its  southern  parts, 
the  Mississippi  River  would  immediately  begin  to  flow  with 
o-reater  swiftness,  and  so  too  all  the  streams  which  are  tribu- 
tary  to  it  would  have  their  energy  enhanced  up  to  the  foot  of 
their  mountain-torrents.  On  the  other  hand,  if  the  continent 
sank  down  a  hundred  feet,  all  these  streams  would  at  once  be- 
come less  effective  agents  of  erosion  and  transportation.  We 
thus  see  that  all  the  erosive  work  of  the  land  is  to  a  greater  or 
less  extent  determined  by  what  is  called  the  principle  of  base 
level  of  erosion.  This  principle,  first  distinctly  suggested  by 
J.  W.  Powell,  has  been  amplified  by  other  American  geolo- 
gists and  has  served  to  bring  into  clear  light  the  peculiar 
sensitiveness  of  our  streams  to  the  position  of  the  sea  or  of 
hard  layers  in  the  rocks  which  control  the  inclination  of  their 
stream-beds. 

We  must  now  turn  our  attention  to  another  mode  in  which 
water  wears  away  the  valleys  of  streams.  So  far,  we  have 
considered  only  that  portion  of  the  rain  which  flows  over  the 
surface  of  the  ground,  but  it  needs  only  a  moment's  notice  to 


I  66  ASPEC7S   OF  THE  EARTH. 

show  us  that  this  is  only  one  element  of  the  rainfall.  If  we 
watch  any  ordinary  soil-covered  portion  of  the  earth's  surface 
in  a  time  of  rain,  we  observe  that  a  considerable  portion  of  the 
water,  an  amount  which  varies  with  the  amount  of  water  which 
falls  in  a  given  time  and  the  porosity  of  the  surface,  enters 
into  the  around.  This  subterranean  or  soil  water  passes  for  a 
great  distance  beneath  the  surface  of  the  earth.  In  this  jour- 
ney, the  underground  water  plays  a  very  different  part  from 
that  performed  by  the  superficial  streams.  Except  in  the 
rare  cases  where  it  forms  distinct  caverns,  it  slowly  creeps  on 
its  way  downward  to  the  sea,  never  attaining  a  speed  of  mo- 
tion which  gives  it  any  cutting  power  whatsoever  ;  but  in  this 
underground  journey  it  becomes  in  most  cases  charged  with 
carbonic-acid  gas  and  is  thus  enabled  to  dissolve  more  or  less 
of  the  rocks  through  which  it  passes.  Finally  this  under- 
ground water  emerges  into  the  open  air  and  journeys  through 
the  streams  to  the  sea,  conveying  much  dissolved  matter  taken 
from  the  rocks  through  which  it  passes.  Through  the  action 
of  this  underground  water,  all  the  rocks  for  a  considerable 
depth  below  the  surface  are  constantly  diminishing  in  volume, 
tiny  crevices  are  formed  between  their  grains,  and  the  weight 
of  the  superincumbent  matter  in  most  cases  causes  the  strata 
to  press  these  crevices  together  almost  as  fast  as  they  are 
formed.  This  action  is  particularly  conspicuous  near  the 
surface  of  the  ground,  within  the  limits  of  a  few  score  feet  in 
depth.  The  result  is  that  in  every  river-valley  we  have  the 
whole  area  gradually  down-sinking  by  subterranean  erosion. 
A  portion  of  this  matter,  broken  up  by  the  action  of  pene- 
trating water,  remains  as  the  soil-covering,  but  the  interstitial 
decay  and  the  removal  of  the  matter  go  on  for  great  depths 
beneath  the  soil.      So  hidden   is  this  process  that  even   those 


RIVERS  AND    ]'ALLErS. 


167 


well  trained  in  such  observations  may  not  note  its  effects,  but 
careful  inquiry  exhibits  some  very  conspicuous  results  of  its 
operation.  In  the  Southern  States  of  this  country,  it  is  often 
possible  to  observe  a  layer  of  limestone,  say  five  feet  in  thick- 
ness, which  at  one  point  has,  by  some  impervious  overlying 
deposit,  been  protected  from  the  action  of  penetrating  waters. 
A  few  hundred  feet  away  we  may  find  the  same  bed  exposed 
to  this  percolating  erosion   of  water.      At  such  points  we  ob- 


Diagram  Showing  the  Successive   Stages  of  Erosion  in  a  Valley  Underlai/I   by  Tilted  Rocks  of  Varying 

Hardness. 
(Note  how  the  streams,  at  first  near  each  other,  are  separated  as  they  wear  downward.) 

serve  that  the  limy  matter  has  been  to  a  great  extent  removed 
from  the  layer  of  rock,  leaving  only  the  clay  or  sand  which 
may  have  been  commingled  with  it.  In  this  case,  the  layer 
will  always  be  greatly  diminished  in  thickness  ;  what  was 
originally  a  bed  five  feet  thick  may  become  a  layer  not  more 
than  one  foot  in  depth,  though  the  bed  may  in  other  respects 
retain  its  original  form. 

We  observe  that  this  interstitial  erosion  of  rocks  goes  on 
in  a  greater  or  less  measure  over  all  parts  of  the  river-valley. 
Thus,  while  a  stream-bed  is  exposed  to  the  actual  cutting 
which  the  superficial  portions  of  the  river  may  bring  about, 


1 68  ASPECTS   OF  THE  EARTH. 

all  portions  of  its  valley  are  wearing  down  by  the  interstitial 
decay.  It  will  be  observed  in  the  cut  on  page  167,  which 
shows  a  section  crossing  a  river-valley,  that  we  have  in  such  a 
basin  two  distinct  topographic  features.  There  is  a  channel, 
which,  as  we  readily  see,  was  carved  by  the  flowing  stream. 
On  either  side,  leading  up  to  the  divide  which  separates  the 
river  from  the  next  stream,  is  a  more  or  less  gentle  slope 
across  a  wide  field  of  country.  In  the  main,  the  downward 
wearing  of  this  side  slope  is  accomplished  by  the  percolating 
waters  in  the  manner  before  noted.  To  conceive  the  forma- 
tion of  a  river-valley,  the  observer  must  in  his  imagination 
combine  the  action  of  these  erosive  agents  working  on  the 
surface  and  in  the  under  earth.  He  must  imagine  an  ordi- 
nary river  to  consist  not  only  of  the  main  channel,  but  of 
many  tributary  streams  branching  like  the  limbs  of  a  great 
fan-shaped  tree.  Each  of  these  branches  is  slowly  swinging 
to  and  fro,  driven  about  by  the  wrestle  with  its  alluvial  mate- 
rial. In  time,  every  portion  of  the  valley  is  crossed  again  and 
again  by  the  bed  of  some  stream  in  its  serpentine  swings  to 
the  right  and  left  of  its  present  path. 

It  will  be  well  for  the  student,  when  standing  in  some 
river-valley  of  normal  structure,  such  as  that  of  the  Ohio,  or 
in  other  river-valleys  south  of  the  glacial  belt,  to  imagine  a 
vertical  line  extending  from  the  present  surface  to  the  height 
of  a  mile  above  that  level.  He  should  then  try  to  picture  to 
himself  the  endless  wandering  of  the  streams  in  their  conflict 
with  the  detritus  which  encumbers  their  beds.  He  must  con- 
ceive that  the  brooks  or  rivers  which  are  nearest  the  vertical 
line  have  again  and  again  swung  to  and  fro  across  its  path.  If 
he  could  restore  to  the  surface,  layer  by  layer,  every  part  of 
material  which   had  been  taken  away,  and  bring  to  their  an- 


RIVERS  AXD    VALLEYS.  1 69 

cient  positions  all  the  several  stream-beds,  he  would  find  his 
line  again  and  again  intersected  by  them.  The  time  in  which 
the  stream-beds  lay  over  the  given  vertical  would  be  but  brief. 
Perhaps,  if  it  were  possible  to  make  an  actual  diagram  of 
their  position  and  duration,  with  reference  to  the  given  ver- 
tical Ime,  we  should  find  that  not  more  than  one-fiftieth  of 
its  space  was  occupied  by  the  channels  of  the  old  brooks  or 
rivers.  All  the  intermediate  space  not  so  occupied  by  the 
channels  indicates  the  interstitial  erosion  effected  by  under- 
ground water. 

The  distribution  of  rivers  upon  the  surface  of  the  earth 
depends  upon  a  variety  of  circumstances,  of  which  the  more 
important  are  the  amount  of  rainfall  and  the  attitude  of  the 
mountain-built  rocks  of  the  country  on  the  surface  of  the  con- 
tinent or  other  lands  occupied  by  the  stream.  In  general 
rivers  oriorinate  in  the  mountainous  sections  of  the  lands  and 
flow  thence  down  the  slopes  of  the  table-lands  which  surround 
the  mountains  to  the  sea.  The  size  of  a  river  as  well  as  the 
direction  of  its  flow  depend  in  a  measure  upon  the  exist- 
ence of  table-land  districts  and  the  inclination  which  they 
have.  The  oreater  rivers,  such  as  the  Amazon  and  the  Mis- 
sissippi.  are  found  where  two  or  more  sets  of  table-lands  slope 
together  to  the  central  parts  of  the  continental  area.  In  these 
conditions  streams  from  each  mountain  district  coalesce  and 
form  a  great  river.  Generally  the  principal  stream,  as  for  in- 
stance the  main  Mississippi,  lies  at  no  great  height  above  the 
sea-level.  Thus,  in  the  uprising  and  downsinkings  of  the  con- 
tinent, which  in  the  course  of  geological  ages  are  frequent,  the 
main  stream  is  sometimes  during  the  periods  of  depression  an 
arm  of  the  sea,  and  again  in  times  of  elevation  becomes  a  river, 
while  the  higher  lying  tributaries  are  more  rarely  if  ever  suf- 


I  JO  ASPECTS   OF  THE  EARTH. 

fused  by  the  ocean  waters.  The  Mississippi  River  has  proba- 
bly at  several  times  thus  been  destroyed,  the  tributaries  from 
the  sea  and  west  discharging  into  a  great  arm  of  the  Mexican 
o-ulf,  formed  during  the  subsidence  of  the  continent.  Hence 
it  comes  about  that  the  upper  waters  of  such  a  river  system 
commonly  flow  in  deep  gorges  which  they  have  been  carving 
for  many  geological  periods,  while  the  channel  of  the  main 
stream  frequently  covered  by  ocean  waters  flows  in  an  indis- 
tinct and  recently  carved  depression. 

The  peculiar  distribution  of  mountains  and  their  accom- 
panying table-lands  about  the  several  oceans  causes  the 
amount  of  river  waters  contributed  to  these  areas  to  vary  in 
a  remarkable  manner  ;  thus  the  continents  about  the  Atlantic 
have  their  highlands  so  disposed  that  their  waters  drain  to- 
wards that  ocean.  At  least  nine-tenths  of  the  river  water  from 
North  and  South  America,  the  larger  part  of  that  from  Africa, 
and  the  whole  of  that  from  Europe  pour  into  the  Atlantic 
basin  or  into  the  seas,  the  Arctic  or  Mediterranean  system, 
which  directly  communicate  with  the  Atlantic.  It  is  probable 
that  more  than  one-half  the  rain-water  which  flows  from  the 
land  escapes  into  this  relatively  small  division  of  the  oceanic 
areas.  The  Indian  Ocean  receives  the  water  from  several 
o-reat  streams  in  southern  Asia  and  some  of  lesser  importance 
which  flow  from  Africa,  but  its  share  of  oceanic  land  waters  is 
very  much  less  considerable  than  that  of  the  Atlantic.  The 
only  very  large  streams  which  find  immediate  access  to  the 
Pacific  waters  are  those  of  eastern  Asia.  From  America  only 
three  considerable  streams  pour  into  the  Pacific,  viz.,  the 
Yukon,  the  Columbia,  and  the  Colorado,  none  of  which  are 
to  be  ranked  with  the  greater  rivers  of  the  world. 

There  are   certain    important  geological  consequences  de- 


RIVERS   AXD    VALLEFS.  17 1 

pendent  upon  this  peculiar  arrangement  of  the  rivers.  The 
undissolved  sediment  borne  out  b)'  the  streams  in  the  form 
of  visible  mud,  owing  to  the  influence  of  gravity  upon  the 
particles  of  lock  material,  quickly  finds  its  way  to  the  bottom. 
Hence,  it  arises  that  the  Atlantic  basin  is  the  seat  of  a  very 
much  more  extensive  sedimentary  deposit  than  the  other  parts 
of  the  oceanic  areas.  It  is  only  the  perfectly  dissolved  mate- 
rial, that  which  does  not  appear  to  the  eye,  which  can  journey 
far  in  currents  of  the  sea.  It  is  to  these  marine  currents  we 
owe  the  incessant  transfer  of  the  dissolved  mineral  material 
poured  into  the  Atlantic,  to  the  other  great  ocean  realms. 


Diagram  Showing  Gravel  Terraces,  each  Marking  a  Stage  of  Downcutting  by  a  River. 
(The  dotted  part  of  the  section  shows  alluvial  material  ;  the  straight  lines  the  bed-rock.) 

In  order  to  aid  the  reader  in  forming  a  conception  as 
to  the  history  of  a  river-valley,  a  cut  is  given  which  shows 
in  a  diagrammatic  way  the  process  by  which  a  river-valley 
wears  downward.  On  the  basis  of  fact  presented  in  this 
fiorure,  it  will  be  well  for  the  observer,  bv  the  use  of  his  con- 
structive  imagination,  to  frame  a  picture  of  the  past  history 
of  any  considerable  system  of  land  waters.  If  this  image 
is  well  brought  to  mind,  he  will  have  attained  one  of  the 
greatest  conceptions  which  geology  offers  to  its  votaries. 

The  foreeoinor  considerations  will  enable  the  reader,  in  a 
general  way,  to  conceive  the  laws  under  which  a  river-system 


172  ASPECTS    OF  THE  EARTH. 

is  developed   and   maintained.      It   is   necessary,   however,   in 
order  to  complete  the  picture,  to  set  before  him  certain  acci- 
dents which  may  happen  in  the  history  of  a  stream.      In  the 
case  of  a  river-basin  such  as  that  of  the   Ohio,  a  basin  which 
we  frequently  take  for  illustration  for  the  reason  that  it  is  one 
of  the  most  normal  of  all  those  on  the  American  continent, 
the   natural  history  of  the  stream   is  as  follows:    When  the 
land  which  now  constitutes  this  great  valley  first  came  above 
the  ocean,  it  was  a  region  of  great  plains,  on  which  flourished 
the  dense  swamps  of  the  Carboniferous  era.     Through  this 
plain,  the  streams  seem  for  a  time  to  have  wandered  deviously, 
with  undetermined  channels.      Gradually,  as  the  Appalachian 
and  other  mountains  developed,  and  the  slopes  of  the  streams 
increased,   they   carved    themselves    channels,  —  the    general 
course   of  these  channels  being  determined  to  a  certain    ex- 
tent by  the  inclination  of  the  rocks.     As  the  Alleghanies  rose 
hio-her  and  the  table-lands  on  their  banks  came  to  a  greater 
elevation  above  the  sea,   the  organization  of  the  main  river 
and  its  tributaries  was  made  more  and  more  complete.     If  the 
continent  should  continue  for  some  geological  periods  without 
any  change  in  the  level  of  the  sea,  the  mountain  brooks  would 
gradually  carve  down  the  hills  in  which  they  lie,   the  table- 
lands would  slowly  disappear,  and  the  surface  would  return  to 
its  primeval  state  of  a  great  swamp.     The  rocks  beneath  this 
swamp   would  be  subjected  only   to    interstitial   or  corrosive 
decay,   for  the  reason   that  the  streams  would  not  have  fall 
enough  to  work  upon  their  beds  by  mechanical  erosion.     In 
proportion   as  the  lands  of  the  valley  were  high  above    the 
sea,  the  erosive  effect  of  their  waters  would  have  great  effect. 
With  every  foot  of  diminished  height  above  the  ocean-level, 
the  energy  of  erosion    would    decrease,   while  the    corrosive. 


J^IVERS   AXD    VALLEYS.  1/3 

or  underground,  wearing  would  remain  more  nearly  stead- 
fast. 

It  is,  from  the  foregoing  considerations,  easy  to  see  that 
the  ratio  between  the  erosion  and  corrosion  effected  by  the 
rainfall  in  a  river-basin  determines,  in  a  very  important  way, 
the  aspect  of  that  region.  Whereas,  in  the  Ohio,  the  total 
descent  of  the  waters  in  their  great  distance  of  flow  is  rela- 
tively small,  corrosion  may  nearly  overtake  the  erosive  down- 
wearing,  and  so  the  general  level  of  the  country  will  be 
brought  down  almost  to  the  river-channel,  the  main  stream 
being  bordered  by  a  line  of  low  escarpments  on  the  margin 
of  its  alluvial  plains. 

For  a  contrast  with  the  conditions  presented  by  the  Ohio, 
where  the  rainfall  throughout  the  valley  is  great,  where  the 
elevation  of  the  region  is  slowly  brought  about,  and  therefore 
the  corrosion  relatively  considerable,  let  us  turn  to  the  case 
of  the  lower  Colorado,  where  the  stream  flow^s,  for  some  hun- 
dreds of  miles,  through  a  country  which  has  a  very  small  sup- 
ply of  rain  and  where  it  receives  very  trifling  tributaries  and 
where  the  surface  of  the  country  has  risen  rapidly  from  the 
sea.  The  head-waters  of  the  Colorado  in  the  Rocky  Moun- 
tains are  fed  by  the  considerable  snowfall  of  that  region  ; 
these  melting  snows  maintain  a  powerful  current  through  the 
channel  of  the  stream  at  all  seasons  of  the  year.  The  result 
is,  that,  while  the  region  on  either  side  of  the  Colorado  has 
been  rapidly  elevated  during  the  last  geological  periods,  there 
has  been  no  proportionate  corrosion  of  the  rocks  on  either 
side  of  that  stream.  The  bordering  lands  have  remained  for 
many  geological  ages  little  affected  by  underground  water  or 
the  to  and  fro  swingings  of  the  lesser  streams.  The  conse- 
quences of  this   peculiar  position   is  that  the  Colorado  flows 


174  ASPECTS    OF  THE  EARTH. 

through   a  great  canon,  which,  in   places,  has  the  depth   of  a 
mile. 

Between  the  conditions  of  the  Colorado  canon  and  those 
of  a  valley  such  as  the  southern  part  of  the  Ohio  basin  ex- 
hibits, we  have  every  degree  of  divergence  of  aspect,  and  the 
slope  of  the  drainage  basin  toward  the  gorge  of  the  stream 
indicates  in  a  general  way  the  relative  intensity  of  the  erosive 
and  corrosive  forces.  There  is  a  peculiar  effect  arising  from 
the  diverse  hardness  of  horizontal  strata  in  a  river-valley, 
which  deserves  note  in  this  part  of  our  inquiry.  Wherever  it 
has  a  very  hard  bed  underlaid  by  softer  strata,  this  hard  bed 
at  first  makes  a  precipice  next  the  bank  of  the  stream.  If  the 
underlying  bed  be  so  little  resisting  that  the  weather  wears  it 
rapidly  away,  it  will  often  decay  with  such  speed  that  the 
steep  face  will  be  driven  backward  across  the  country  until  it 
finally  appears  in  the  form  of  an  isolated  table-land  as  is  shown 
in  the  cut.  Finally,  when  this  table-land,  decaying  on  its 
several  sides,  has  been  reduced  much  in  area,  it  may  appear 
in  the  form  of  what  is  called  in  the  Mississippi  Valley  a  butte. 
Such  retreat-escarpments  are  often  very  conspicuous  and 
beautiful  features  in  the  landscape.  Excellent  examples  of 
such  structures  occur  in  horizontally  disposed  strata  on  both 
sides  of  the  Mississippi  and  in  the  Saxon  Switzerland,  where 
they  afford  the  table-like  rocks  of  that  beautiful  district — iso- 
lated eminences,  which,  in  that  region  of  ancient  warfare,  are 
often  crowned  by  fortresses.  Such  buttes,  or  tables  of  rock, 
only  occur  where  the  strata  of  a  river  valley  lie  in  a  horizon- 
tal attitude  and  where  hard  beds  and  soft  are  interminofled. 
Where  the  rocks  of  varied  hardness  depart  very  much  in  their 
attitudes  from  the  horizontal,  they  greatly  affect  the  flow  of 
the  stream  as  it  wears  down  its  bed,  in   the  manner  indicated 


Canon    of    trie     Via    Mala,    Switzerland 
<Sho\ving  the  work  done  by  a  large  torrent  on  rocks  of  close  texture  which  are  readily  eroded  by  the  stream.) 


RIIERS   AXD    VALLLYS.  175 

by  the  accompanying  figures.  Thus  the  position  of  a  stream 
in  a  valley  where  the  rocks  are  steeply  inclined  is  determined 
by  the  various  inclinations  of  the  strata. 

All  normal  rivers  where  they  discharge  into  the  sea 
construct  more  or  less  extensive  terrace-like  deposits  which 
receive  the  name  of  deltas.  These  accumulations  are  made 
up,  in  the  main,  of  the  detritus  which  finds  its  way  to  the 
bottom  as  soon  as  the  flow  of  the  river-water  is  arrested  by 
its  mergence  in  the  sea.  In  part  they  are  composed  of  the 
remains  of  marine  animals  and  plants  which  are  more  or  less 
al:)undantly  developed  on  the  mud  flats  at  the  debouchure 
of  the  stream.  The  deposition  of  the  mud  from  tlie  river- 
waters  is  more  quickly  effected  than  would  otherwise  be  the 
case,  through  the  peculiar  effect  which  the  salt  in  the  w^ater 
has  in  throwing  down  suspended  materials.  Those  wdio  are 
familiar  with  the  operations  of  the  chemical  laboratory  know- 
how  much  more  readily  fine  sediment  is  precipitated  when 
the  solution  contains  a  little  saline  matter.  Hence  it  comes 
about  that  the  river  mud  rarely  appears  in  the  sea  at  any 
great  distance  from  the  mouth  of  the  stream.  Certain  great 
rivers,  such  as  the  Amazon,  throw  out  such  a  vast  tide  of 
fresh  water  that  it  drifts  away  for  scores  of  miles  before  it 
becomes  mingled  with  the  heavier  water  of  the  ocean.  As 
soon,  however,  as  the  waves  have  churned  it  into  mixture  with 
the  salt  water,  the  mud  quickly  finds  its  way  to  the  bottom. 

A  large  part  of  the  mud  brought  out  by  a  river  thus  falls 
near  the  land  ;  thence  much  of  it  is  driven  back  by  the  waves 
aofainst  the  shore,  and  throuorh  this  embarrassment  of  alluvium 
the  river  has  to  struggle  with  frequent  changes  of  its  current. 
All  the  very  great  rivers  of  the  world  have  these  delta  dis- 
tricts in  which  the  stream  has  endlessly  to  struggle  with  the 


176  asfjlCts  of  the  earth. 

embarrassment   caused  by  the  waste  derived  from   the  more 
inland  coimtry. 

Althoueh  in  its  eeneral  character  the  delta  district  is  a 
continuation  of  its  alluvial  terraces  which  bound  the  stream 
from  the  time  it  passes  out  of  the  mountain-torrents  into 
the  river,  this  district  next  the  sea  always  presents  certain 
important  peculiarities,  which  have  had  in  many  regions  a 
noteworthy  influence  on  the  destiny  of  the  peoples.  The 
broad  low  lands  of  the  delta  afford  very  imperfect  bounda- 
ries to  the  current  of  the  river.  The  stream  is  continually 
chancring  its  path  in  its  endeavors  to  find  less  obstructed  ways 
to  the  sea.  Sometimes  these  changes  of  channel  take  place 
with  great  rapidity  and  in  a  very  frequent  manner  ;  the  result 
is  that  the  delta  districts  are  commonly  cut  up  into  many 
distinct  islands  of  varied  size,  separated  from  each  other  by 
swift  or  languid  currents  according  as  their  paths  are  recently 
constructed  or  are  in  process  of  abandonment  by  the  stream. 
These  separate  islands,  cut  off  the  one  from  the  other  by 
deep  channels  and  often  divided  by  extensive  morasses,  afford 
natural  strongholds  more  suited  to  the  development  of  nascent 
civilizations  than  the  steep-walled  natural  fortresses  which 
abound  in  most  inland  countries.  The  soil  of  these  delta 
islands  is  commonly  very  fertile.  The  streams  generally 
abound  in  fish,  and  thus  the  inhabitants  of  such  districts  are 
readily  provided  with  means  of  subsistence.  Moreover,  these 
estuaries  of  rivers  generally  communicate  freely  with  the  sea, 
and  navigation,  beginning  at  first  with  small  boats,  readily 
extends  until  it  takes  on  the  form  of  oceanic  commerce. 
Holland,  the  deltas  of  the  Rhine,  of  the  Nile,  and  those  of 
a  number  of  Asiatic  rivers,  are  thus  the  sites  of  very  ancient 
and    prosperous    peoples  who   attained  a   considerable   civili- 


ri]w:rs  axd  valleys.  177 

zation  before  the  interior  districts,  less  fertile,  more  subject 
to  the  incursion  of  enemies,  and  deprived  of  access  to  the 
sea,  attained  any  thing  like  a  high  place  in  the  arts  of  life. 

So  far  we  have  considered  the  history  of  a  stream  where 
it  has  been  left  free  from  all  natural  interference  to  develop- 
ment. In  such  conditions,  its  basin  is  shaped  as  the  con- 
currence of  the  erosive  and  corrosive  forces  may  determine. 
In  fact,  few  river-basins  enjoy  any  such  immunity  from  dis- 
turbing conditions.  Their  sensitive  streams  are  variously 
affected  by  geological  influences  of  an  external  sort.  As 
these  invading  forces  profoundly  affect  the  form  of  river- 
valleys,  we  may  take  a  glance  at  their  nature.  The  most 
common  disturbing  influence  which  may  affect  a  river-valley 
of  considerable  area  arises  from  the  construction  of  mountain- 
ridges  across  the  path  of  its  streams.  It  was  once  supposed 
that  mountains  were  suddenly  formed.  It  is  now  clear  that 
in  most,  if  not  in  all,  cases  they  have  gradually  grown  to 
their  present  height.  Now,  as  the  greater  number  of  our 
mountains  lie  in  the  paths  of  streams  which  existed  before 
the  elevations  were  formed,  it  follows  that  our  rivers  which 
intersect  mountain-ridges  have  had  to  wrestle  with  the  bar- 
riers produced  by  the  elevations.  It  may  in  cases  have  hap- 
pened that  the  ridge  or  wall  of  a  mountain  has  been  suddenly 
uplifted  across  the  path  of  a  stream,  but  in  most  of  the  cases 
where  we  can  trace  the  history  of  the  contention  between 
ridge  and  stream,  we  find  that  the  elevation  has  been  formed 
with  such  slowness  that  the  river  has  kept  open  its  channel 
across  the  line  of  the  developing  obstruction.  This  leads  us 
to  the  conclusion  that  mountains  are  never,  to  any  extent, 
barriers  to  the  path  of  rivers  ;  they  probably,  in  most  cases, 
grow  so  gradually  that  the  streams  may  keep  their  ways  open 


178 


ASPECTS    OF  THE  EARTH. 


through  the  obstacle  which  they  tend  to  interpose.  The  part 
which  mountains  play  in  the  history  of  rivers  is  thus  limited 
to  a  narrower  field  than  we  should  at  first  suppose.  They 
aff"ect  the  path  of  rivers  by  changing  the  inclination  of  rocks 
and  so  directing  the  swing  of  the  streams.      They  also  serve 


Dunkar  Spiti,   India. 
(Showing  mountain  wall,  talus  leading  to  valley,  and  stream  embarrassed  by  debris.) 

to  maintain  the  torrential  portion  of  a  river-system,  and  so 
afford  a  ground  whence  the  stream  may  obtain  the  alluvium 
necessary  to  make  the  plains  which  border  the  lower  part  of 
its  course.  As  we  have  seen,  a  chemical  action  which  goes 
on  in  the  material  of  these  delta-districts  serves  an  important 
purpose  in  the  economy  of  the  earth's  surface.      Were  it  not 


RIVERS   AXD    VALLEYS. 


1/9 


for  the  continuance  of  the  mountain-building  forces,  the  tor- 
rents, owing  to  the  rapid  down-wearing  of  their  beds,  would 
soon  cease  to  afford  such  detrital  material.  The  combined 
machinery  of  torrent  and  mountain  so  operates  as  to  maintain 
the  supply  of  detritus  required  by  the  needs  of  the  sea  for  the 


(Showing  the  distributiun  of  the  torrents  of  the  upper  part  of  the  valley.) 

maintenance  of  organic  life  in   its  depths  and  for  the  deposi- 
tion of  strata  on  its  floor. 

There  are  other  and  more  formidable  geologic  agents 
tending  to  modify  river-basins  ;  the  chief  of  these  are  glaciers. 
When  a  glacial  period  comes  upon  a  country,  the  sheets  of 
ice  are  first  imposed  upon   the  mountain-tops,  and  thence  the 


l8o  ASPECTS    OF   THE  EARTH. 

ice  creeps  down  the  torrent- and  river-beds  far  below  tlie  snow- 
line, in  a  manner  now  seen   in  Switzerland  and   Norway.     As 
long  as  the  ice-streams  follow  the  old  torrent-channels,  they 
act  in   something  like   the   fashions  of  the    flowing  waters,  to 
gouge  out  the  rocks  and  deepen  the  valleys  ;  but  as  the  glacial 
period  advances  and  the  ice-sheet  spreads  beyond  the  moun- 
tains, enveloping  the  plains  as  well,  when  the  glacier  attains 
the  thickness  of  thousands  of  feet,   it  disregards  the  valleys 
in  its  movement  and  sweeps  on  in  majestic  march  across  the 
surface  of    the  country.      As  long  as  the  continental  glacier 
remains,   its    tendency  is    to    destroy  the  river-valleys.     The 
result  of  this  action  is  to  plane  down  the  whole  land,   and, 
to   a  certain  extent,  to  destroy  all   pre-existing   river-systems. 
During  the  last  glacial   period,  the  old   river-valleys  were  to  a 
great  degree  worn   away,  and  the  remaining  portion   of  their 
troughs  was  to  a  considerable  extent  buried  beneath  a  thick 
coating  of  debris  which  the   ice   had  worn  from  the  surface  of 
the  land  and  dropped  upon  that  surface  as  it  retreated.      The 
result  is  that   in   all  countries  which  were  affected  by  the  last 
glacial  period,  the  river-valleys  have  only  here  and  there,  and 
in  all  cases  imperfectly,  returned  to  their  ancient  beds.      Ever 
since  the  ice  went  away,  they  have  been  engaged  in  a  struggle 
to  restore  their  ruined  ways.      As  yet,  this  work  is  most  imper- 
fectly accomplished,  and  even   if  a  glacial   period   should   not 
return   to  the    northern    part   of    North   America   for  several 
million  years,  the  task  of  restoring  the  river-systems  to  their 
original  aspects  would  not  be  completed. 

We  see  a  simple  indication  of  this  confusion  of  the  old 
drainage  brought  about  by  glacial  action,  in  the  vast  number 
of  lakes  lodged  within  depressions  of  the  surface  in  New 
England  as  well  as  in  all  parts  of  the  glaciated  district.      We 


RIVERS  AND    VALLEFS. 


i8l 


have  only  to  compare  the  valley  of  such  a  stream  as  the  James 
River,  which  lies  south  of  the  glacial  belt,  with  a  New  Eng- 
land valley,  such  as  that  of  the  Merrimack,  to  see  the  impor- 
tance of  the  effects  accomplished  by  a  glacial  sheet  on  the 
river-system.  The  valley  of  the  James  is  entirely  without 
lakes ;  every  part  of  its  area  slopes  downward  toward  the 
sea.  In  the  valley  of  the  Merrimack,  there  are  hundreds  of 
these  water-basins.      A  very  large  part  of  its  surface  is  occu- 


'"  ■''■JitM4te%:iiii'fii#ii!i»^^^^^ 


Norwegian  Fjord. 
(Showing  the  form  of  a  valley  shaped  by  glacial  action.) 

pied  by  lakes,  which  owe  their  origin   to   irregularities  of  the 
surface,  produced  by  the  last  glacial  period. 

There  is  yet  another  way  in  which  rivers  may  be  naturally 
obstructed  ;  this  is  by  lava-streams  pouring  out  into  their 
valleys.  In  all  volcanic  regions,  the  river-beds  are  apt  to 
receive  great  inundations  of  such  material.  When  gigantic 
eruptions  of  lava,  such  as  have  occurred  in  the  recent  geo- 
logical periods  in  Oregon  and  California,  in  Southern  India, 
and  in  Eastern   Europe,  are  poured  out,  the  stream-beds  are 


l82 


ASPECTS    OF  THE  EARTH. 


apt  to  be  gorged  with  this  igneous  material,  it  may  be  for  a 
distance  of  a  hundred  miles  from  the  volcanic  vents.  At  first 
the  river  is  dried  up  by  the  fiery  torrent.  When  the  lava  cools 
it  becomes  solid,  often  much  more  resisting  to  water-action 
than  the  rocks  originally  underlying  the  stream.  It  generally 
happens  that  the  lava-current  is  higher  in  the  middle  of  its 
course  than  it  is  upon  the  margin.  The  result  is  that  when 
the  river  begins  again  to  flow  its  course  is  divided  into  two, 
part  of  the  water  flowing  on  either  side  of  the  lava-stream. 
As  time  goes  on  and  the  streams  cut  deeply  into  their  new 
beds,  they  may  leave  the  old  lava-mass  perched  upon  a  hill. 


Diagram  Showing  Old  River  Channels  on  Top  of  Hills. 

(The  upper  dark  layer  shows  lava,  covering  recent  stream-beds  ;   the  faint  lines  show  the  topography 
when  the  lava-streams  flowed.) 


as  shown  in  the  diagram.  It  happens  in  California  that  these 
streams  occupied  by  the  lava  contain  gold-bearing  sands,  some- 
times in  very  large  quantities.  The  deposits  of  gold  were 
accumulated  before  the  lava  came  into  the  ancient  river-beds. 
Miners  have  learned  that  wherever  a  mass  of  lava  occupies 
the  position  indicated  in  the  diagram  they  may  reasonably 
expect,  by  excavating  through  the  side  of  the  hill,  to  strike 
the  old  river-channel,  and  beneath  the  cap  of  lava  to  find 
large  deposits  containing  gold,  which  they  may  win  more 
easily  than  the  deposits  in  the  beds  of  the  existing  streams. 
Owing  to  the  extensive  explorations  which  have  been  made 
in  this  search  for  gold  in  such  positions,  we  have  gained  some 


RIJ'JlRS  AXD    VALLEFS.  183 

very  important    information    from   these   obliterated,    encum- 
bered river-beds. 

Perhaps  the  oldest  evidences  which  we  have  of  prehistoric 
man  have  been  obtained  from  these  mines  driven  into  the 
ancient  channels  of  rivers  on  the  Pacific  coast.  A  number 
of  rude  stone  implements  have  been  disinterred  by  these 
mining  operations,  which  clearly  prove  that  the  region  was 
extensively  occupied   by   man      One    human    skull    has    also 


V.^iCiMi^  ^J.-.;ks  III  trie  Valley  of  the  Puerco 

(Showing  extent  of  erosion  in  the  surrounding  plateau  ;  the  sharp  hills  are  the  necks  ot  old  volcanoes, 
the  cones  of  which  have  been  worn  away  by  the  river  action  ! 

been  found  in  these  workings,  along  with  the  remains  of 
several  extinct  animals.  The  streams  flow  on  either  side 
of  the  old  lava-current,  and  as  they  cut  but  slowly  into  the 
subjacent  rock,  we  are  able  with  safety  to  infer  that  these 
remains  of  man  have  been  in  existence  for  twenty  thousand 
years  or  more.  In  Central  France,  near  by  the  town  of  Le 
Puy,  similar  lava-streams  also  contain  buried  human  remains. 
In  both  these  cases,  the  remains  of  man  have  been  found 
associated  with  those  of  extinct  animals  ;  which  fact  serves 
to  show  that  the  conclusion  we  draw  as  to   the  antiquity  of 


1 84  ASPECTS   OF  THE  EARTH. 

man,  from  the  erosion  which  has  taken  place  since  the  lava- 
current  flowed,  is  well  founded. 

Although  the  rivers  have  to  maintain  a  battle  with  many 
obstructing  actions  due  to  natural  causes,  there  are  only  two 
circumstances  derived  from  the  revolutions  of  the  earth's 
surface  which  seriously  affect  their  history,  at  least  in  a  per- 
manent way.  Where  the  rainfall  of  a  country  undergoes 
considerable  variations,  as  appears  always  to  be  the  case  in 
the  course  of  long  geological  periods,  the  streams  necessarily 
find  their  volume  diminished  or  increased,  sometimes  in  an 
important  degree.  However  much  the  rainfall  may  vary,  the 
architecture  of  a  river,  the  position  of  its  branches,  the  distri- 
bution of  its  torrent  and  alluvial  sections  generally  remain 
essentially  unchanged.  Even  where  the  continent  on  which 
a  river  lies  is  greatly  elevated  beyond  its  original  height,  the 
system  of  the  streams  remains  as  it  was  before.  Thus  our 
rivers  are  in  many  cases  the  oldest  features  on  the  earth's 
surface.  The  upper  waters  of  the  Tennessee,  for  instance, 
especially  those  of  the  French  Broad  River,  have  apparently 
endured  since  the  earliest  ages  of  which  we  have  any  distinct 
record  in  the  great  stone  book.  They  seem  to  have  flowed  at 
the  beginning  of  the  Cambrian  time,  and  their  channels  have 
borne  their  floods  to  the  sea  during  periods  in  which  the  conti- 
nent of  North  America  has  underorone  vast  chansfes  in  form. 
Certain  groups  of  fishes,  such  as  the  gar  pikes,  which  proba- 
bly had  their  cradle  in  these  waters,  have  apparently  dwelt  in 
them  continually  since  the  Devonian  time. 

The  only  conditions  which  actually  lead  to  the  destruction 
of  a  river-system  arise  either  from  the  imposition  of  a  glacial 
sheet  on  the  surface  of  a  country  or  from  its  submergence 
beneath  the  level  of  the  sea.     We  have  already  seen  that  the 


RIVERS  AND    VALLEFS.  185 

interruption  brought  about  by  a  continental  glacier  on  the 
streams  in  the  country  over  which  it  extends  is  usually  but 
temporary.  In  a  like  manner,  the  submergence  of  a  great 
valley  beneath  the  sea-level  is  not  apt  entirely  to  destroy  its 
basin.  When  the  surface  of  the  continent  recovers  its  posi- 
tion, returning  to  the  state  of  dry  land,  there  is  generally 
enough  left  of  the  form  of  the  basin  to  cause  the  stream,  at 
least  in  a  general  way,  to  follow  its  ancient  paths. 

With  the  foregoing  brief  sketch  of  their  mechanism,  we 
will  turn  our  attention  to  the  relations  between  the  civilization 
of  man  and  the  system  of  the  rivers.  Nowhere  else  in  the 
physical  machinery  of  our  earth  is  the  influence  of  the  hand 
of  man  so  well  shown  as  in  the  conditions  of  rivers.  Nowhere 
else  are  his  destructive  or  conservative  powers  so  important. 
The  effect  of  man's  action  upon  rivers  is  in  the  main  due  to 
the  fact  that  his  occupancy  of  the  earth  leads  to  the  removal 
of  its  forest  covering.  We  have  already  incidentally  noted 
the  relation  of  trees  to  the  immediate  bounds  of  a  stream  ;  we 
have  seen  that  the  woods  are  continually  pressing  upon  the 
margins  of  a  river,  causing  it  to  sway  to  and  fro,  and  tending 
always  to  narrow  its  channel.  This  is  only  one,  and  perhaps 
the  least  important,  of  the  effects  exercised  by  forests  on  the 
regimen  of  the  greater  streams.  It  is  necessary  to  consider  the 
action  of  forests  over  the  whole  basin  of  a  river,  in  order  to  see 
the  magnitude  of  their  influence  on  the  action  of  these  waters. 

The  valleys  of  most  rivers  are  forest-clad.  Whether  these 
forests  have  the  gigantic  growth  characteristic  of  fertile  dis- 
tricts in  the  tropics  and  the  temperate  zones,  or  take  the 
shape  of  stunted  woods  such  as  extend  far  toward  the  poles, 
they  in  all  cases  form  beneath  their  branches,  and  above  the 
soil,  a  thick,  spongy  coating,  which  affords  a  natural  reservoir 


1 86  ASPECTS   OF  THE  EARTH. 

for  the  rain-waters.  In  most  regions,  this  forest-sponge  has 
a  depth  of  more  than  a  foot  ;  it  not  infrequently  attains  a 
thickness  of  two  feet  or  more.  It  can  commonly  take  into  its 
interstices  a  rainfall  of  three  or  four  inches  in  depth,  or  from 
one-sixth  to  one-tenth  the  ordinary  annual  supply.  This 
water  is  slowly  yielded  to  the  brooks  ;  it  often  requires  weeks 
for  a  single  torrential  rain  entirely  to  escape  into  the  open 
channels  which  bear  It  to  the  sea.  Moreover,  the  fallen 
trunks  and  branches  of  the  trees  clog  the  forest-shaded  rivu- 
lets, making  little  pools,  which  serve  still  further  to  restrain 
the  outo-olno-  of  the  waters.  Our  beavers,  at  one  time  the 
most  widely  distributed  of  our  larger  animals,  at  first  making 
avail  of  these  natural  ponds  formed  by  fallen  timber,  learned 
in  time  to  construct  more  artful  dams  so  as  to  retain  exten- 
sive basins  of  water.  Thus,  In  the  natural  condition  of  the 
North  American  rivers,  as  well  as  those  of  most  other  coun- 
tries, before  man  began  to  clear  away  the  forests,  the  woods 
constituted  a  great  system  of  reservoirs,  in  which  the  rains 
were  retained  into  the  period  of  Intervening  droughts.  In 
this  state  of  the  surface,  the  main  channels  of  a  river-system 
were  continually  the  seat  of  streams  of  moderate  flow.  These 
channels  were  no  wider  than  was  required  by  the  rate  at  which 
these  forest-impounded  waters  escaped. 

When  man  resorted  to  the  soil  as  the  source  of  his  food, 
he  began  to  clear  away  the  forests  and  by  tillage  to  destroy 
the  spongy  covering  of  the  earth  which  they  created.  With 
the  advance  of  civilization,  all  the  great  valleys  on  the  north- 
ern temperate  zone  have  been  to  a  considerable  extent 
deprived  of  their  forest-covering.  In  this  new  state  of  the 
surface,  the  rain-water  is  no  longer  held  back  as  it  was  of  old, 
but  flows  quickly  over  the  surface  of  the  soil  and  enters  the 


RIVERS   AND    VALLEYS.  187 

water-ways.  The  result  is  that  all  the  old  channels  bear,  in 
times  of  flood,  a  body  of  water  far  greater  than  that  which  was 
put  into  them  before  the  forests  were  cleared  away.  They 
have  been  compelled  to  widen  their  channels  by  cutting  away 
a  strip  of  the  alluvial  land  on  either  side.  Thus,  in  the  case 
of  the  Ohio  River,  the  bed  occupied  by  the  flood-waters  has, 
since  the  beginning  of  the  present  century,  been  widened  to 
the  amount  of  about  one-fifth  of  its  total  diameter.  Despite 
this  widening,  it  is  now  unable  to  bear  away  the  flood-waters 
yielded  to  it  by  the  extensive  tilled  surfaces  of  its  basin.  In 
times  of  flood  it  rises  higher  than  of  old  and  spreads  devas- 
tation over  a  wider  area  of  the  alluvial  plains.  In  times  of 
drought  the  stream  shrinks  within  its  waste  of  encumbering 
sands  and  becomes  unnavigable.  — , 

In   the  present  condition  of  the   Mississippi  valley,   these      ! 
floods   and   droughts   seriously   affect    the    interests    of    man. 
There,  as  in   all   other  civilized  countries,   the  great  seats  of 
population  tend   to  gather  on  the  river-banks.     The  alluvial 
lands  are  in  all  cases  singularly  fertile  ;  and  the  streams  them- 
selves  afford  natural  ways  of  transportation,  the  value  of  which       .  (J\ 
does  not  seem  to  become  lessened  by  the  great  extension  of    / 
railway  systems.      In  the  present  condition  of  these  valleys,     ' y' 
the   fitness  of   these  streams   for  navigation   is  progressively 
diminishing,    for   both    in    times   of   flood   and    in    periods   of 
drought  they  are  unsuited  to  the  uses  of  commerce.      More- 
over, in  the  flood  periods,  the  streams  are  a  very  serious  men- 
ace to  all  the  towns  which  are  gathered  along  the  river-banks. 
As  yet,  we  have  only  seen  the  beginning  of  these  evils  ;  for 
notwithstanding  the  extensive  settlements  in  the  Mississippi 
valleys,  more  than  half  their  original  forest-covering  remains. 
When,   with    the   rapid   increase   of    population,    these   river- 


i88  ASPECTS   OF  THE  EARTH. 

basins  become  as  thoroughly  subjected  to  the  uses  of  man  as 
are  those  of  Europe,  we  have  yet  greater  ills  to  apprehend. 

The  problem  of  the  Mississippi  valley  is  one  of  national 
importance.  By  far  the  greater  part  of  the  food-producing 
capacity  of  our  continent  lies  in  the  basin  of  that  great  system 
of  rivers.  It  is  therefore  worth  our  while  to  consider  the 
method  by  which  this  area  can  best  be  brought  to  serve  the 
needs  of  man  without  imposing  a  serious  burden  on  his  arts. 
Although  it  is  impossible  in  these  few  pages  to  consider  the 
way  in  which  this  great  task  may  be  accomplished,  it  is  perhaps 
worth  while  to  note  the  general  conditions  which  have  to  be 
met  in  this  and  other  great  valleys  if  that  end  is  to  be  secured. 

In  endeavoring  to  meet  the  evils  which  arise  from  the 
removal  of  forest-covering  from  the  surface  of  a  country,  we 
find  that  the  difficulties  to  be  considered  are  as  follows : 
First,  those  which  arise  from  the  diminished  restraint  put 
upon  the  movements  of  the  water  which  comes  to  the  earth's 
surface  in  times  of  heavy  rain  or  of  melting  snow.  Next,  the 
evils  due  to  the  rapid  wasting  of  the  soil,  which,  in  its  unpro- 
tected condition,  is  readily  washed  into  the  stream-beds.  The 
first  of  these  evils  gives  rise  to  serious  destruction  of  wealth 
and  to  the  interruption  of  industries.  The  second  threatens 
the  loss  of  that  precious  soil-covering  on  which  depends  the 
relation  of  all  land  life,  that  of  plants  and  man  and  beast,  to 
the  surface  of  the  earth.  It  is  clearly  evident  that  we  cannot 
hope  to  preserve  any  considerable  portion  of  our  forest  lands 
from  destruction.  The  need  of  subsistence  such  as  is  drawn 
from  the  soil  is  immediate  and  overwhelming.  During  the 
last  century,  Europe  has  been  able  to  preserve  a  portion  of  Its 
forests,  and  indeed  to  win  extensive  areas  back  to  the  condi- 
tion of  woods,  for  the  reason  that  it  could  draw  supplies  of 


RIVERS   AXD    VALLEYS.  1 89 

food  from  this  country  ;  but  when  our  American  soils  are  occu- 
pied, it  does  not  seem  Hkely  that  other  parts  of  the  world  will 
afford  any  such  opportunity  for  obtaining  foreign  grain.      At 
most,  we  may  expect  that  a  small  area,  perhaps  not  exceeding 
one-tenth    of   our  original   forests,    may   be   retained   in   their 
present  shape,   in   order   to  afford   supplies  of   timber.      It   is 
therefore  necessary,  if  we  have  to  control  these   flood-waters 
at  all,  to  devise  some  means  by  which  we  may   Imitate  the  old 
natural    system   of  water  storage  which   the    primeval  woods 
afforded.     There  is  but  one  method  by  which  this  end  may  be 
accomplished,  viz.:  by  creating  artificial  reservoirs  in  which  the 
waters  may  be  for  a  time  retained  during  the  period  of  floods. 
Some    years    ago,   a    distinguished  engineer,   Mr.   Charles 
EUet,   suggested  a    system  of  controlling  the    floods    of    the 
Mississippi  valley.       He    proposed   to  build  certain   dams  in 
the  upper  waters   of  the   Mississippi   system,  in  which,  during 
the  times  of  flood,  a  considerable  part  of  the  flow  might  be 
impounded,  to  be  discharged   into  the  channels  at  such  times 
as  was  needed  to  maintain  a  navigable  depth  of  water.      There 
are  certain  objections  to  the  details  of  the  system  proposed  by 
Mr.  Ellet,  the  principal  of  w^hich  is  that  the  existence  of  very 
large  reservoirs  would  add  another  source  of  danger  to  those 
which  the  floods  now  inflict  upon  the  valleys  of  these  streams. 
It  is  difficult  cheaply  to  build  retaining  dams  so  that  they  may 
be  absolutely  secure  from  the  risks  of  giving  way.     The  burst- 
ing of    such   a  dam   in   time  of  flood  might  prove  peculiarly 
disastrous." 


*The  foregoing  statements  concerning  the  danger  of  clams  in  river-channels 
were  written  some  years  ago,  and  were  then  based  on  general  considerations,  as  well 
as  on  the  experience  which  had  been  gathered  from  numerous  minor  disasters  from 
the  bursting  of  reservoirs. 


190  ASPECTS   OF  THE  EARTH. 

It  seems,  however,  possible  that  a  shght  modification  of 
Mr.  EUet's  phm  would  more  effectively  accomplish  the  end 
he  had  in  view,  without  creating  the  risks  above  noted.  For 
in  place  of  half  a  dozen  great  artificial  lakes,  we  should  adopt 
the  plan  of  having  many  thousands,  or  tens  of  thousands,  of 
smaller  reservoirs,  so  arranged  that  no  one  would,  by  its 
bursting,  lead  to  the  destruction  of  any  other.  We  could  by 
this  means  retain  on  the  surface  of  the  land  a  very  consider- 
able part  of  the  flood-waters  which  now  prove  disastrous 
to  the  valleys  below.  Computations,  which  it  would  be  out 
of  place  to  present  in  a  writing  of  this  nature,  have  shown 
me  that  it  would  apparently  be  possible,  with  an  expendi- 
ture of  less  than  fifty  million  dollars,  to  diminish  the  rise  of 
floods  at  Cincinnati  to  the  amount  of  at  least  twelve  feet, 
and  at  the  same  time  secure  to  that  river  a  good  degree  of 
navigability  during  the  whole  of  the  dry  summer  season. 
To  control  in  a  similar  manner  the  floods  which  ravage 
the  valleys  of  the  other  large  tributaries  of  the  Mississippi, 
would  perhaps  require  a  total  expenditure  exceeding  one 
hundred  million  dollars.  The  maintenance  of  this  system 
would  necessarily  be  costly;  it  would  perhaps  amount  to 
as  much  as  ten  million  dollars  a  year.  It  seems,  however, 
possible  that  for  this  cost  we  might  obtain  a  substantial 
immunity  from  the  worst  destruction  accomplished  by  our 
floods.  Even  if  this  system  should  be  adopted,  it  would  be 
necessary,  decade  by  decade,  as  the  process  of  forest  removal 
advanced,  to  extend  still  further  the  area  of  the  storage 
reservoirs. 

While  the  proper  control  of  the  Mississippi  drainage 
system  is  of  great  importance  to  the  nation  at  large,  to  the 
states   which  border   upon   its  waters  it  is  a  matter  of  vital 


RIVERS   AXn    VALLEYS.  191 

necessity.  Whether  this  great  task  is  to  be  undertaken  by 
the  Federal  Government  or  by  associated  commonwealths, 
there  can  be  no  question  that  it  should  be  at  once  entered 
upon.  Every  year  increases  the  magnitude  of  the  necessities 
and  the  difficulty  of  devising  means  to  meet  them. 

The  great  disaster  in  Pennsylvania  which  occurred  on  the 
I  St  of  June,  1889,  afforded  a  startling  illustration  as  to  the 
dangers  which  may  attend  on  the  storage  of  large  masses  of 
water  in  our  river-valleys.  To  understand  the  measure  of 
this  danger  we  must  remember  that  every  stream-bed  is  ad- 
justed by  the  processes  which  lead  to  its  construction  to  the 
carriage  of  an  amount  of  water  which  it  is  required  to  bear 
by  the  ordinary  processes  of  nature.  When,  as  in  the  Penn- 
sylvania accident,  a  great  body  of  water  is  by  the  swift  de- 
struction of  a  dam  delivered  to  the  channel,  it  with  diffi- 
culty moves  down  the  insufficient  water-way.  assuming  as 
it  sweeps  onward  almost  the  form  of  a  wall.  The  impetu- 
osity of  the  current  causes  it  to  rend  from  the  path  it 
pursues  a  vast  quantity  of  stones  and  mud,  which  so  thicken 
the  fluid  and  encumber  its  way  that  it  does  not  flow  as 
water  ordinarily  does,  but  acts  rather  like  an  avalanche  of 
snow  or  ice.  The  wall-like  character  of  the  flood's  advance 
may  be  retained  for  scores  of  miles  below  the  point  where 
it  is  delivered  to  the  valley.  Unlike  ordinary  floods,  the 
tide  does  not  rise  gradually,  but  comes  with  a  single  over- 
whelming blow. 

It  happens  that  the  disaster  above  referred  to  was  due 
to  the  outbreak  of  a  mass  of  water  which  was  held  in  its 
dangerous  position,  not  to  meet  any  grave  public  need,  but 
for  the  purposes  of  sport.  It  is  easy  to  dispose  of  such 
cases    by  well-enforced    laws   which  will  prevent  the    enclos- 


192  ASPECTS   OF  THE  EARTH 

lire    of    fish-ponds  of  any   such  size  as  to  endanger  lives  or 
property  in    the  valley  towards   which   the  waters  discharge. 
It  must  be  recognized,  however,  that  in  all  our  river-valleys 
it  is  necessary  to  store  in  reservoirs   large  quantities  of  water 
which  serve  important  public  needs.     For  water  power,  for  the 
supply  of  cities,  or  for  the  maintenance  of  navigation  in  our 
rivers,  it  will    be  necessary  to    construct  many    such    basins. 
Althouo-h  there  must  be  always  some  element  of  danger  wher- 
ever  these  reservoirs  exist,  the  risks  may  easily  be  reduced  to 
a  minimum.      Rarely    indeed    is    it  warrantable    to  have  the 
depth  of  the  water  exceed  twenty  feet  in  the  part  of  the  basin 
next  the  dam.      If  under  these   conditions  the   storage   is  in- 
sufficient, a  number  of  dams  should  be  constructed,  one  above 
the  other.       It  is  the    habit  of    our    engineers   frequently  to 
construct   dams  where  the  retaining-wall  is  relatively  thin,  the 
o-reater  portion   of  the  barrier  being  composed  of  earth.      It 
frequently  happens  that  these  masses  of  earth  become  thor- 
oughly water-soaked.     In  such  cases   it  is  often  a  mere  pulpy 
mass,  like  quicksand,  which    adds    but  little  to  the  strength 
of  the  dam.      If  now  the  single  wall  is  ruptured,  the  whole 
obstruction    may  readily  break    away.      Two    good    masonry 
dams  a  sufficient  distance  apart,  well  founded  on  rock  beds, 
the  interspace  filled  with  compact  earth  and  the  upper  side 
protected  by  a  sloping    bank  of    dense   clay,  will    commonly 
secure  the  structure  against  any  mischance  save  that  which 
a  great    earthquake    might    bring    to    it  ;  but,  with  any  con- 
ditions of   engineering  care,   no  such  vast    reservoirs  as   Mr. 
Ellet  proposed  should  be  constructed. 

Although  the  American  theory  of  government  looks  to  the 
initiative  of  the  individual  for  the  most  of  the  acts  which  in 
other  lands  are  accomplished  by  the  state,  it  still  has  to  con- 


RIVERS   A^D    VALLEYS.  193 

fess  that  certain  classes  of  work  are  only  accomplishable  by 
federal  control.  Our  great  river  is  fast  becoming-  a  common 
enemy  of  our  people  ;  it  is  our  duty  to  restrain  its  ravages 
as  we  would  those  of  any  other  foe  of  the  state.  During 
the  present  century  the  forest  area  of  the  Ohio  valley  has 
probably  been  diminished  by  the  amount  of  one-fourth  of  its 
original  area.  More  than  this  proportion  of  the  country  has 
been  cleared  of  its  wood  ;  but  in  certain  districts,  as  on  the 
Green  River  in  Kentucky  and  in  portions  of  Tennessee  and 
other  States,  the  woods  have  gained  upon  areas  which  were 
previously  prairie  land.  The  result  is  that  probably  three- 
fourths  as  much  forest  now  exists  as  when  the  country  was 
first  settled.  With  the  rapid  advance  in  population  it  seems 
quite  certain  that  by  the  middle  of  the  next  century  the  forest 
area  will  be  reduced  to  less  than  one-fourth  of  the  extent  it 
now  occupies.  The  effect  of  this  change  upon  the  magnitude 
of  the  floods  cannot  fail  to  be  great. 

There  is  another  economic  and  social  problem  connected 
with  rivers,  which  is  just  receiving  attention  in  this  country, 
though  it  has  long  occupied  the  minds  of  engineers  and 
statesmen  in  other  lands.  This  is  the  question  of  irrigation 
to  be  applied  to  the  desert  fields  of  the  region  in  and  about 
the  Cordilleras  of  North  America.  In  the  present  state  of  the 
rainfall  in  the  Rocky  Mountain  districts  and  the  neighboring 
portions  of  the  continent,  the  soil  is  generally  unfit  for  agri- 
culture on  account  of  the  scanty  rainfall  during  the  warmer 
part  of  the  year.  The  rain  generally  falls  in  winter  season, 
and  passes  from  the  surface  of  the  country  through  the 
steeply  inclined  river-beds  before  the  season  of  growth 
approaches  ;  the  result  is  that  the  earth  is  occupied  by  a 
scanty  herbage  which  gives  an   indifferent  support  to  cattle. 


194  ASPECTS   OF  THE  EARTH. 

and  refuses  to  reward  any  form  of  tillage.      It  is  hardly  too 
much  to  say  that  one-third  of  the  area  of  the  United  States, 
excluding   Alaska,    is    more   or  less    completely   sterilized  by 
summer  droughts,  and  a  large  portion  of  the  remaining  area 
suffers    grievously  from  frequent  dearth  of    rain.      There    is 
but  one  way  of  mitigating  this   evil,  but  one  way  in  which  a 
large  part  of  this  soil  can  be  fitted  for  the  plough,  and  that  is 
by  storing  a  portion  of  the  winter  rains  in  reservoirs,  lead- 
ino-  the  water  forth  at  the  fit  time  upon  the  adjacent  lands 
by   means     of     irrigation  canals.       The    general    preliminary 
studies  of    the  cordilleran   district   made  under  the  direction 
of  Major  J.  W.  Powell,  director  of    the   United  States  Geo- 
logical Survey,   have  served  to  show  that  a  large  part  of  this 
field  is  winnable  to  tillage  by  the  simple  and   relatively  inex- 
pensive method  of  storing  and  distributing  the  natural  rain- 
fall of  the  country.      There  seems  a  chance  that  we  may  gain 
to   acrriculture  from  these  desert   lands  a  region  having   the 
food-producing    power  of    at    least     six    States    the    size    of 
Illinois.      It    seems    likely    that    by  the    application    of    this 
beneficent  system  we  may  secure  in  the  magnificent  climate 
of    that  elevated    and  salubrious  country  the    food   for   from 
ten  to  twenty  million  people. 

In  large  part,  this  irrigation  system  may  be  accomplished 
without  the  construction  of  reservoirs  to  retain  the  winter 
waters  ;  but  the  efi'ective  execution  of  this  project  will  require 
the  construction  of  very  many  artificial  reservoirs  for  the 
storao-e  of  water  accumulated  in  time  of  rain,  to  be  made  of 
service  in  the  dry  season.  At  first  sight  it  may  seem  that 
such  a  system  promises  to  give  rise  to  disasters  such  as  that 
which  overwhelmed  the  city  of  Johnstown  and  the  neighbor- 
ing villages  of  the  Conemaugh  valley.      Although,   as  before 


Filling's    Cascade,    Utah. 
1  Showing  type  of  waterfall  in  massive  rocks  which  are  cut  by  joints.     U.  S.  Geological  Survey. 


RIVERS  AND    VALLEYS.  1 95 

remarked,  all  reservoirs  are  in  their  way  dangerous, — as  for 
that  matter  are  most  of  our  engineering  works,  when  care- 
lessly administered, — there  are  particular  reasons  why  a  system 
of  storage  basins  in  the  Rocky  Mountains  can  be  constructed 
with  much  less  danger  than  in  other  regions.  In  the  first 
place,  long-continued  torrential  rains  such  as  lead  to  the 
Conemaugh  accident  are  uncommon  in  the  Rocky  Mountain 
district.  Cloud-bursts  occur  in  particular  districts  ;  but  they 
are  of  brief  duration,  generally  limited  to  very  small  areas. 
It  is  generally  easy  in  that  country  to  choose  sites  where 
short,  well-founded  dams  will  retain  the  water  over  laro-e  areas 
in  such  circumstances  as  would  lead  to  no  great  disaster  in 
case  the  barrier  were  broken.  Moreover,  the  country  is  now 
essentially  unfit  for  the  uses  of  man  except  in  the  temporary 
work  of  mining;  and  even  if  disasters  were  frequent,  while  in 
fact  they  are  likely  to  be  but  of  seldom  occurrence,  we  could 
maintain  that  the  interests  of  man  were  well  served  even  if 
the  population  made  possible  to  the  country  by  irrigation 
were  to  a  certain  extent  endangered. 

The  lessons  of  experience  clearly  point  to  care  in  the 
selection  of  the  sites  to  be  occupied  by  storage  reservoirs 
which  are  intended  to  serve  the  irrigation  works  of  the 
Rocky  Mountains  ;  the  lessons  indeed  should  be  heeded  in 
all  countries.  In  the  cordilleran  district,  however,  there  is 
reason  to  hope  that  the  construction  of  these  dams  may  be 
under  the  supervision  of  able  engineers,  who  will  guard  the 
folk  to  be  from  the  ravages  of  these  unnecessary  floods. 
Although  the  dangers  are  manifest,  the  means  of  avoiding 
them  are  equally  clear  :  it  requires  only  the  courage  to  spend 
the  money  required  to  secure  safety,  to  make  the  proposed 
irrigation    basins  of    cordilleras  as  safe   as   good  railways  or 


196  ASPECTS   OF  THE  EARTH. 

well-constructed  houses.  The  lesson  derived  from  the  Coiie- 
mauofh  disaster  is  not  that  we  should  avoid  such  enLrineerin-jf 
works  as  brought  about  this  disaster,  but  that  we  should  take 
pains  to  secure  the  public  against  the  mischances  which  faulty 
construction  may  bring  upon  them. 


THE  INSTABILITY  OF  THE  ATMOSPHERE. 


Contrast  between  Conditions  of  Ocean  and  Air  ;  Mingling  of  these  Elements  ;  Dependence 
of  Organic  Life  upon  them.— Maintenance  of  Temperature  ;  Conditions  of  other  Bodies 
in  Solar  System  ;  Conditions  of  Moon  ;  Way  in  which  Heat  is  Retained  ;  Conditions 
of  Temperature  in  Past  Geologic  Ages.-Evidence  of  Long-Continued  Equilibrium  in 
Atmosphere  both  in  Heat  and  Constituents.-Elimination  of  Oxygen  and  Carbon  from 
Air  ;  Methods  of  Replacement.— Circulation  of  the  Air  ;  Cause  of  Movements  ;  Tropical 
Winds  ;  Trade  Winds  ;  their  Permanence  ;  their  Origin.— Direction  of  Movement  of 
Trade  Winds  ;  Cause  of  Inclination  to  the  Equator.— Effects  of  Irregular  Distribution  of 
Heat ;  Compensating  Influence  of  Ocean  Currents.— Inconstant  Winds  ;  their  Origin  ; 
Land  and  Sea  Breezes  ;  East  Wind  of  Atlantic  Coast.— Variable  Winds  ;  their  Origin  ; 
Experiments  ;  Cause  of  Whiriing  Movement  ;  Various  Conditions  of  Origin.— Torna- 
does ;  Origin  of  ;  Effects  of.— Speed  of  Movement  and  Appearance  of  Tornadoes  ; 
Origin  of  Destructive  Action  ;  Width  of  Path  ;  Ways  of  Avoiding  Accidents.— Distribu- 
tion of  Tornadoes  ;  Effects  on  Forests.— Cyclones  ;  their  Origin  ;  How  Ships  may  Avoid 
them.— Effects  of  Cyclones  on  Shore  ;  in  Bay  of  Bengal  ;  in  Florida.— General  Economy 
of  Atmospheric  Movements. 

The  solid  and  relatively  fixed  mass  of  the  earth  is  wrapped 
about  by  two  great  envelopes,  the  atmosphere  and  the  waters, 
each  characterized  by  a  certain  instability.  The  water-enve- 
lope is  mainly  gathered  into  the  basins  of  the  seas,  where  it 
has  definite  boundaries  and  a  distinct  uppermost  surface. 
Still,  a  small  portion  of  the  water  is  constantly  in  the  air  ;  or, 
proceeding  from  the  air  to  the  earth,  is  making  an  often  long- 
continued  and  roundabout  journey  over  or  through  the  super- 
ficial parts  of  the  earth's  crust  on  its  way  back  to  the  seas. 
All  our  rocks  contain  a  portion  of  water  on  its  way  to  the 
ocean,  or  temporarily  imprisoned  in  their  interstices  ;  so  we 
may  fairly  regard  the  water   of  the   earth   as  constituting  an 


198  ASPECTS   OF  THE  EARTH. 

envelope  of  its  whole  surface,  though  the  greater  portion  of 
the  substance  is  in  the  sea-basins.  The  envelope  of  the  air  is 
also  somewhat  peculiarly  distributed  over  the  earth's  surface, 
but  the  irregularity  is  much  less  pronounced  than  in  the  case 
of  the  water. 

If  the  water  came  to  a  state  of  rest,  it  would  all  return  to 
the  seas  and  lakes,  and  would  cover  only  three-fourths  of  the 
earth's  surface  ;  and  under  the  same  conditions  of  rest  the  air 
would  cover  the  whole  earth,  but  it  would  be  densest  where  it  lay 
on  the  surface  of  the  sea,  and  thinnest  over  the  surface  of  the 
land.  These  two  envelopes  are  somewhat  commingled  ;  the 
water  is  more  or  less  mixed  with  the  air  and  with  the  solid 
parts  of  the  earth,  and  the  air  is  to  a  certain  extent  commin- 
gled with  the  water  and  enters  even  as  much  as  the  water 
into  the  interstices  of  the  rocks.  Both  these  envelopes  are 
capable  of  taking  some  part  of  the  other  substance  into  their 
masses,  but  they  differ  much  in  the  measure  of  this  capacity. 
Water  can  take  a  large  amount  of  solid  matter  into  suspen- 
sion by  dissolving  it,  while  the  air  can  only  receive  and  retain 
foreign  matter  when  that  matter  is  in  the  state  of  gas.  We 
might  very  much  extend  this  list  of  related  and  contrasted 
properties  of  the  two  great  oceans,  but  for  our  purpose  we 
need  to  note  only  the  last  and  most  important  feature  of  con- 
trast. The  air  is  gaseous  ;  it  is  normally  composed  of  several 
commingled  gases,  while  the  water  is  a  f^uid  having  a  more 
definite  constitution  and  containing  other  substances  in  a 
somewhat  unessential  way. 

All  the  possibilities  of  organic  life  which  the  earth  pre- 
sents, and  which,  so  far  as  we  can  conceive,  any  other  sphere 
can  afford,  depend  upon  the  coincidence,  on  the  surface  of  a 
sphere,  of  these  contrasted  and  yet  related  masses  of  air  and 


THE  IXSTABILITV  OF  THE  ATMOSPHERE.  199 

water.  It  is  true  that  other  materials,  such  as  carbon,  are  also 
among-  the  necessary  conditions  of  organic  development ;  but, 
though  these  mineral  substances  are  found  everywhere  in  the 
physical  universe,  they  can  only  come  into  conditions  where 
they  may  enter  upon  the  form  of  living  beings  when  they 
are  associated  with  the  enveloping  oceans  of  air  and  water. 
Where  these  envelopes  are  wanting,  as  on  the  surface  of  the 
moon,  the  sphere  remains  without  the  possibilities  of  life. 
Even  where  these  envelopes  may  happen  to  exist,  it  is  only 
with  the  conjunction  of  certain  temperatures  that  life  can 
possibly  develop.  If  the  heat  at  the  surface  of  the  sphere 
remains  below  the  freezing-point,  or  if  it  attains  a  temperature 
exceeding  150°  F.,  the  conditions  of  life  disappear.  Although 
the  organic  form  of  matter  depends  upon  the  conjunction, 
on  the  surface  of  a  planet,  of  water,  air,  and  a  certain  temper- 
ature, the  dependence  upon  the  air  appears  to  be  the  most 
immediate,  for  to  that  element  we  owe  not  only  the  oxygen, 
but  also  the  preservation  of  the  temperature  which  makes  life 
possible. 

The  maintenance  of  the  temperature  necessary  for  organic 
life  on  the  earth's  surface  is  a  problem  of  singular  difTficulty. 
In  the  spaces  between  the  planets  we  have  a  temperature  of 
several  hundred  degrees  below  the  zero  of  Fahrenheit,  and  in 
the  sun  a  temperature  which  is  probably  to  be  measured  by 
tens  of  thousands  of  degrees.  The  difficulty  was  to  preserve 
on  the  surface  of  the  earth  a  temperature  which  should 
remain,  over  the  most  of  that  surface,  through  all  the  geologi- 
cal ages,  above  the  freezing-point  of  water,  and  yet  below  the 
temperature  of  one  hundred  and  fifty  degrees.  We  see  the 
immediate  effect  of  this  combination  of  air  and  water  when 
we  consider  the  condition  of  the  moon's  surface.      That  sphere 


200  ASPECTS    OF   J  HE  EARTH. 

is  without  either  atmosphere  or  oceans,  yet  in  many  other 
regards  is  much  Hke  our  earth  ;  but  owing  to  this  want  of  the 
envelopes  of  air  and  water  it  has  remained  a  perfect  desert. 
The  heat  flies  away  from  it  as  fast  as  it  is  received  from  the 
sun  ;  even  during  the  long  day  it  is  douJDtful  if  the  temper- 
ature of  the  moon's  surface  rises  above  zero  of  Fahrenheit, 
and  in  the  night  it  probably  falls  to  near  the  temperature  of 
space,  a  hundred  degrees  or  more  below  the  point  which  is 
ever  attained  on  the  earth. 

All  those  who  become  keenly  interested  in  the  final  condi- 
tions of  the  earth's  surface  find  themselves  naturally  led  to 
exercise  their  constructive  imaginations  in  conceiving  the  con- 
ditions of  other  spheres  than  our  own.  It  is  therefore  inter- 
esting to  note  that  the  atmospheric  envelope  appears  to  be  a 
feature  common  to  all  the  planets  of  our  solar  system  which 
are  near  enough  to  afford  us  opportunities  for  observation. 
The  sun  itself  has  a  vast  atmosphere,  though  this  envelope  is 
intensely  heated,  and  contains  many  substances  in  the  form  of 
vapor  which  are  solid  bodies  on  our  earth.  The  planet  Venus 
appears  to  have  an  atmospheric  envelope,  and  is  surcharged 
with  cloud  in  such  a  measure  that  only  one  or  two  lofty  ele- 
vations project  above  the  enduring  field  of  vapor.  Mars 
evidently  has  an  atmosphere  of  considerable  density,  which 
apparently  yields  from  its  clouds  frequent  falls  of  snow. 
These  snow-falls  may  be  observed  in  the  proper  season  extend- 
ing far  south  from  either  pole.  Jupiter  and  Saturn  appear 
also  to  have  deep  envelopes  of  air. 

The  only  well-ascertained  exception  of  the  principle  that 
the  spheres  of  our  planetary  system  are  wrapped  by  atmos- 
pheric coverings  is  found  in  the  case  of  our  own  satellite.  Al- 
though many  observers  have  fancied  they  beheld  phenomena 


THE  IX STABILITY  OF  THE  ATMOSPHERE.  201 

indicating  a.  trace  of  such  an  envelope  on  the  moon,  it  now 
appears  clear  that  there  is  no  gaseous  covering  to  this  body. 
If  any  ever  existed,  it  has  been  absorbed  in  the  mass  of  this 
desolate  world.  The  entire  and  ever-abiding  desolation  of 
that  sphere  can  only  be  explained  by  the  absence  of  the  gas- 
eous envelope  on  which  all  planetary  life  must  inevitably 
depend. 

The  atmosphere  serves  to  retain  the  heat  of  the  sun  by 
virtue  of  a  singular  feature  of  its  structure.  The  direct  rays 
of  the  sun  pass  through  it  to  the  surface  of  the  earth  with 
ease,  and  heat  the  superficial  parts  of  the  land  and  sea. 
These  warmed  surfaces  seek  to  discharge  their  heat  directly 
back  into  the  celestial  spaces  by  the  process  of  radiation.  If 
the  way  out  were  as  easily  traversed  as  the  way  in,  the  heat 
received  from  the  sun  would  be  removed  as  fast  as  it  came, 
and  the  earth's  surface  would  remain  at  the  temperature 
of  space  ;  but  the  air  is  a  trap.  The  radiant  heat  from  the 
earth's  surface  cannot  traverse  it  with  the  same  speed  as 
the  direct  rays  from  the  sun  ;  hence  the  layer  of  air  next  the 
earth's  surface  becomes  warm  in  the  measure  which  is  neces- 
sary for  organic  life. 

It  is  not  easy  to  appreciate  the  delicacy  of  adjustment 
which  is  required  to  establish  this  temperature  demanded  by 
organic  life,  and  to  maintain  it  through  the  geological  ages. 
Even  in  the  permanent  heat  of  the  equator,  the  zone  of  life- 
killing  cold  lies  but  four  miles  above  the  surface  of  the  sea. 
As  soon  as  night  comes  on,  this  dead-line  begins  to  descend 
toward  the  surface  ;  by  morning  it  may  have  fallen  to  within 
three  miles  of  the  sea-level.  A  week  of  continued  night  would 
lock  the  tropics  in  a  deadly  frost  and  make  an  end  of  its 
land-life. 


-02  ASPECTS    OF  TEE  EARTH. 

The  oreoloeical  record  shows  us  clearly  that,  in  the  hundred 
million  years  which  have  elapsed  since  the  plants  and  animals 
of  the  land  have  been  in  existence,  the  regions  of  the  tropics 
have   never   been    subjected  to  serious  frost.      From  time  to 
time   durincr   the   course   of   the   earth's   development,  glacial 
periods  have  originated  ice-sheets  about  either  pole.     These 
sheets   of    ice   have    crept    down    toward   the    equator,  often 
attaining    half    the    distance  which    separates   the   regions  of 
o-reatest  cold  from  the  tropics  ;  but  the  intertropical  belt  of 
land   and   sea,  that  great   asylum  whereunto   resorts  the   life 
expelled  from  circumpolar  regions  by  the  glacial  periods,  never 
has  been  subjected  to  a  deadly  temperature.      The  evidence 
that   o-oes  to  show  this  is    simple    and    conclusive.      Certain 
groups  of  plants — as,  for  instance,  the  tree-ferns — and  many 
orders  of  animals  are   extremely  intolerant  of    cold,   yet  the 
fossils  show  us  clearly  that,  from  the  early  geological  ages  to 
the   present  day,   these  forms  have  been    continuously  occu- 
pants of  tropical  districts.      A  very  brief  period  of  cold  would 
have  placed  them    among  the  extinct  creatures  of  the  past. 
An    equally  brief    period  of    heat,   provided    it    brought    the 
atmosphere  and  the  waters  to  a  temperature  above   1 50°  Fah- 
renheit, would  likewise  have  made  an  end  of  organic  life  upon 
the  earth.      It  is  therefore  clear  that  the  atmosphere  is  a  con- 
servator of  heat,  and  that  in  this  conservative  work  it  has  not 
failed  in  its  function  since  the  dawn  of  geological  history.      It 
is  almost  equally  clear  that  the  climate,  in  the  earliest  periods 
of  the  earth's  development  of  which  we  have  any  record  in  the 
rocks,  was,  in  a  general  way,  essentially  like  that  of  the  later 
geological  periods,  and  even  that  of  the  present  day.      In  cer- 
tain peculiar  conditions  glacial  periods  have  now  and  again 
extended  the  ice-sheets  from  the  poles  for  a  considerable  dis- 


THE  INSTABILITY  OF  THE  ATMOSPHERE.  203 

tance  toward  the  equator.  In  the  periods  which  have  inter- 
vened between  these  times  of  glaciation,  the  temperature  of 
high  altitudes  has  permitted  plants  which  were  clearly  sensitive 
to  cold  to  live  in  regions  within  the  Arctic  Circle.  But  apart 
from  these  great  cycles  of  change,  which  give  us  in  succession 
extreme  and  temperate  climates  about  either  pole,  the  evi- 
dence goes  to  show  that  the  temperature  of  the  earth  has  not 
undergone  great  variations. 

If  our  nearest  companions  in  space,  the  planets  of  the 
solar  system,  have  any  organic  life  developed  upon  them, 
they  must  owe  the  conditions  which  permit  such  beings  in 
the  main  to  the  peculiar  organization  of  their  atmosphere. 
Thus  in  the  case  of  Mercury  and  Venus,  which  are  very 
much  nearer  the  sun  than  the  earth,  the  temperature  on  the 
surface  of  those  spheres  would,  save  for  a  possible  compen- 
sation which  its  atmosphere  might  afford,  be  far  too  elevated 
to  permit  organic  life  to  exist.  If  the  atmospheric  condi- 
tions were  the  same  as  those  of  our  own  earth,  then  the 
heat  even  in  the  circumpolar  regions  of  these  planets  would 
probably  attain  at  certain  seasons  of  the  year  a  temperature 
above  that  of  boiling  water,  while  in  their  tropical  districts 
it  would  probably  never  fall  below  that  point.  It  appears, 
however,  as  before  remarked,  that  Venus  is  deeply  cloud- 
wrapped,  and  it  may  be  that  under  the  shelter  of  such  a 
cloud  mantle  the  fierce  rays  of  the  sun  would  be  in  large 
measure  fended  off  from  the  surface  of  the  planet.  In  the 
case  of  the  planets  which  lie  farther  away  from  the  sun  than 
our  own,  it  may  be  that,  owing  to  the  greater  thickness  of 
their  atmosphere,  the  relatively  small  amount  of  the  sun's 
heat  maintains  the  surface  of  the  spheres  in  conditions  which 
would  make  organic  life  possible. 


204  ASPECTS    OF  THE  EARTH. 

There  can  be  no  question  that  this  evidence  leads  us  to 
the  conclusion  that  the  mass  of  the  air  has  remained  essen- 
tially the  same  during  the  period  of  that  inconceivably  endur- 
ing- past  recorded  in  the  fossiliferous  rocks.  Any  consider- 
able change  in  the  volume  of  the  atmosphere,  without  a 
coincident  alteration  in  the  amount  of  heat  it  received,  would 
be  followed  immediately  by  a  change  in  the  temperature  of 
the  surface  on  which  the  air  lies.  Whenever  we  climb  a  con- 
siderable mountain  we  make  a  practical  experience  of  this 
protective  effect  of  the  atmosphere.  For  each  thousand 
feet  of  that  height — that  is,  for  each  considerable  part  of  the 
atmosphere  we  pass  through — we  find  the  average  annual 
temperature  lowered  by  from  three  to  six  degrees.  At  the 
height  of  a  few  thousand  feet  above  the  equator  we  pass 
from  the  tropical  climate,  and  enter  the  zone  where  frosts 
make  many  forms  of  tropical  life  impossible.  A  little  higher 
we  pass  beyond  the  possibilities  of  life  at  all,  and  enter  into 
the  region  sterilized  by  perpetual  cold.  On  the  other  hand,, 
if  we  had  a  basin  excavated  to  the  depth  of  ten  thousand 
feet  below  the  plane  of  the  sea,  in  the  equatorial  belt,  the 
average  annual  temperature  on  its  bottom  would  so  much 
exceed  the  present  heat  of  the  equatorial  lands  at  the  sea- 
level  that  even  the  most  heat-enduring  forms  of  life  would 
find  it  excessive  and  would  perish.  In  other  words,  to  pre- 
serve the  temperature  of  the  tropics  as  it  has  been  preserved 
from  a  remote  period  in  the  past,  the  total  volume  of  the 
air  must  have  remained  for  all  time  about  what  it  is  at  pres- 
ent ;  at  most  it  can  have  undergone  but  slight  changes  in 
volume. 

This  permanence  of  the  atmosphere  is  the  more  surpris- 
ing when  we  consider  not   its  mass    alone  but  also  its   con- 


THE  INSTABILITY  OF  THE  ATMOSPHERE.  205 

stituents.  As  is  well  known,  the  atmosphere  of  our  earth 
consists  in  the  main  of  nitrogen,  a  substance  which  has  com- 
paratively little  direct  relation  to  the  chemical  or  organic 
work  done  upon  the  surface.  This  relatively  inactive  nitro- 
gen amounts  to  about  three-fourths  of  the  weight  of  the  air. 
With  it  are  mingled  two  other  very  important  gaseous  sub- 
stances, which,  unlike  the  nitrogen,  are  of  the  utmost  impor- 
tance to  animal  life,  and  profoundly  affect  the  physical  history 
of  the  earth's  surface  as  well.  These  substances  are  oxygen, 
which  comprises  about  one-fifth  of  the  weight  of  the  atmos- 
phere, and  carbonic  acid,  a  combination  of  one  atom  of  car- 
bon and  two  of  oxygen,  which  exists  in  very  small  quantity 
at  any  one  time  in  the  atmosphere.  At  the  present  time 
the  proportion  of  this  substance  amounts  to  a  very  small 
fraction  of  one  per  cent,  of  the  total  mass  or  weight  of 
the  air.  These  two  gaseous  materials,  oxygen  and  carbonic 
dioxide,  are  constantly  passing  from  the  atmosphere  to  the 
earth's  crust  in  such  large  amounts  that  it  is  very  difficult 
to  understand  how  the  supply  of  them — a  supply  absolutely 
necessary  for  the  important  functions  of  the  atmosphere — 
is  maintained.  Oxygen  enters  into  the  earth  by  the  process 
of  rusting  and  decaying  which  we  see  going  on  in  the  rocks 
about  us,  and  in  many  other  ways  which  are  not  manifest  to 
the  eye.  Whenever  a  metal  rusts,  or  a  rock-mass  decays, 
it  almost  necessarily  happens  that  a  portion  of  this  oxygen 
becomes  imprisoned  in  the  earth's  crust.  The  present  store 
of  oxygen  in  the  atmosphere  by  weight  amounts  to  about 
three  pounds  upon  the  square  inch  of  surface,  or  about  four 
hundred  pounds  to  the  square  foot.  In  the  processes  of 
what  we  call  decay — but  which  we  would  better  term  change 
— which  have   taken  place  since   the   beginning  of    the  geo- 


2o6  ASPECTS   OF  THE  EARTH. 

logical  record,  it  seems  certain  that  far  more  than  the  amount 
of  oxygen  now  present  in  the  atmosphere  must  have  been 
imprisoned  in  the  oxidized  materials  of  the  earth's  crust. 

As  was  long  ago  shown  by  the  distinguished  chemist, 
Henry  Wurz,  a  very  small  amount  of  the  iron  pyrite  contained 
in  the  earth's  crust  would,  in  decomposing,  absorb  all  the 
oxygen  in  the  atmosphere.  The  chemical  actions  which  serve 
to  take  oxygen  from  the  free  air  into  the  prison  of  the  earth's 
crust  are  numerous,  and  the  gates  of  that  prison  are  rarely 
unbarred.  Once  confined  in  the  rocks  there  seems,  practi- 
cally, hardly  any  way  in  which  it  can  be  set  free  again  ;  at 
least  the  possibilities  of  its  escape  are  so  limited,  as  compared 
with  the  imprisoning  actions,  that  we  cannot  look  to  them  for 
an  effective  restoration  of  this  element  to  the  atmosphere.  At 
first  sight  it  may  seem  possible  that  the  atmosphere  at  one 
time  contained  within  itself,  in  a  ofaseous  form,  a  much  larger 
proportion  of  oxygen  than  it  does  at  present.  May  we  not 
suppose  that  all  the  oxygen  which,  in  the  course  of  geological 
time,  has  been  bound  up  in  the  earth  was,  at  the  beginning  of 
that  time,  in  the  atmosphere,  the  original  store  having  gone 
on  decreasing  as  it  was  drawn  upon  to  supply  the  needs  of  the 
underground  actions  ?  But  here,  as  before,  the  evidence  from 
past  life  serves  to  show  us  that  the  chemical  composition  of 
the  atmosphere  has  changed  as  little  as  its  mass.  If  in  the 
early  geological  ages  there  had  been  on  our  earth  an  atmos- 
phere charged  with  oxygen  in  the  measure  which  the  above 
statements  would  require  us  to  suppose,  animals  could  not 
have  breathed  ;  for,  as  experiments  show,  they  are  little  tol- 
erant of  any  material  increase  in  the  proportion  of  this  gas. 
There  is  thus,  from  these  limited  considerations,  a  reason  to 
believe  that  the  insects  and  batrachians  of  the  Carboniferous 


THE  IXSTABILITF  OF  THE  ATMOSPHERE.  207 

period  found  the  air  essentially  the  same  as  that  breathed  by 
their  successors  living  at  the  present  day.  These  considera- 
tions could  be  extended  and  enforced  if  space  were  at  our 
disposal  ;  but  the  reader  may  trust  the  geologist  when  he 
states  that  all  the  evidence  indicates  that  the  atmosphere,  in 
times  even  antecedent  to  the  Carboniferous  period,  did  not 
contain  a  materially  larger  share  of  oxygen  than  it  has  at 
present. 

The  only  way  in  which  we  can  conceive  the  replacement 
of  this  life-giving  oxygen,  which  the  greedy  earth  is  always 
claiming  from  the  air,  is  through  the  action  of  the  plants; 
each  plant,  in  its  process  of  growth,  takes  all  the  carbon  of 
its  woody  matter  from  the  air.  This  carbon  it  finds  in  the 
atmosphere  in  the  form  of  carbonic  dioxide — that  is,  a  chemi- 
cal combination  where  there  is  one  atom  of  carbon  linked 
with  two  atoms  of  oxygen.  Absorbing  this  gas,  it  breaks  up 
the  union  of  the  two  elements,  retains  the  carbon,  and  returns 
the  oxygen  to  the  air.  In  this  way  there  is  a  constant  return 
of  the  precious  life-giving  gas  to  the  atmosphere.  The  car- 
bon is.  it  is  true,  to  a  certain  extent  reunited  with  the  oxygen 
when  the  wood  decays  ;  but  in  part  this  carbon  goes  into  the 
rocks  in  the  form  of  coal  or  limestone,  and  in  so  far  it  effects 
a  substantial  contribution  of  oxygen  to  the  active  supply  on 
which  all  animal  life  depends. 

If  there  were  a  source  whence  a  supply  of  carbonic-acid 
gas  could  be  obtained,  it  would  be  easy  to  explain  the  preser- 
vation in  the  atmosphere  of  both  these  substances  which  are 
so  indispensable  to  organic  life  ;  for  even  the  solar  force  oper- 
ating through  the  plants  would  work  to  break  up  the  union  of 
the  oxygen  and.  the  carbon  composing  this  gas,  and  so  afford 
a  continual  supply  of  these  materials. 


2o8  ASPECTS   OF  THE  EARTH. 

But  now  we  find  ourselves  facing  the  great  mystery  of  the 
atmosphere  :  Whence  comes  this  ever-demanded  store  of 
combined  carbon  and  oxygen?  In  what  manner  is  it  given  to 
the  atmosphere  in  such  a  well-adjusted  measure  that  the  plants 
always  have  their  fit  share  of  carbon,  and  the  animals  never 
any  excess  of  the  oxygen  ?  The  amount  of  this  carbonic 
dioxide  probably  has  never  much,  if  at  all,  exceeded  one  per 
cent,  of  the  atmospheric  mass.  Carbon  is  ever  passing  at  a 
rapid  rate  from  the  air  to  the  earth — our  coal-beds  are  vast 
stores  of  it ;  our  limestones,  composed  in  the  main  of  lime 
carbonate,  contain  far  larger  amounts  than  the  coal  ;  and  in 
the  decay  of  our  crystalline  rocks  vast  amounts  of  it  are  per- 
manently laid  away  out  of  reach  of  the  atmosphere.  There 
can  be  no  doubt  that,  since  life  began  upon  the  earth,  there 
has  been  taken  from  the  air  scores  of  times  as  much  carbon  as 
is  now  contained  in  the  atmosphere.  It  was  once  supposed 
that  this  carbon  was  returned  to  the  air  in  a  regular  and  full 
measure  by  the  action  of  volcanoes.  These  vents  do,  indeed, 
throw  out  a  certain  amount  of  carbonic  acid  as  a  part  of  their 
emanations,  but  it  now  seems  clear  that  they  cannot  begin  to 
maintain  the  balance  against  the  forces  which  tend  to  lock 
carbon  in  the  earth. 

It  was  also  for  a  time  believed  that  the  carbon  now  in  our 
rocks,  placed  there  since  the  beginning  of  organic  life,  was 
originally  all  in  the  atmosphere,  and  that  it  has  gradually 
been  taken  thence  into  the  rocks  of  the  earth ;  but  here 
again  the  fossils  rise  up  and  testify  that  the  air  in  the  most 
ancient  days  of  land-life  did  not  contain  any  such  vast  store 
of  carbonic-acid  gas.  Careful  observations  show  that  the 
ferns  and  other  allies  of  the  plants  which  flourished  in  the 
time  when  the  coal-measures  were  laid  down  will  not  exist  in 


THE  INSTABILITY  OF  THE  ATMOSPHERE.  209 

an  air  containing  a  great  excess  of  carbonic-acid  gas,  and  the 
abundant  air-breathing  animals  of  that  time  certainly  could 
not  have  withstood  any  considerable  increase  of  that  sub- 
stance beyond  what  the  atmosphere  at  present  contains.  We 
are  clearly  justified  in  assuming  that  at  no  one  time  was 
there  in  the  realm  of  the  air  the  hundredth  part  of  the  carbon 
which  is  locked  up  in  the  stratified  rocks.  The  difficult  prob- 
lem before  us  is  to  find  some  source  of  supply  whence  the 
combined  oxygen  and  carbon  can  be  derived  m  uniform  quan- 
tities, as  the  needs  demand.  If  such  a  source  of  supply  could 
be  found,  we  might  then  assume  that  from  it  the  plants,  by 
decomposing  the  elements  of  the  gas,  found  the  source  of  the 
carbon  which  has  been  stored  in  the  earth,  and  that  in  obtain- 
ing this  carbon  they  replenished  the  oxygen  of  the  air. 

Defeated  in  the  effort  of  finding  a  terrestrial  source  of 
carbonic  acid  sufficient  to  supply  the  ever-current  needs  of 
the  atmosphere,  physicists  have  of  late  been  driven  to  the 
hypothesis  that  this  material  comes  upon  the  surface  of  the 
earth  from  the  celestial  spaces.  Dr.  T.  Sterry  Hunt,  in  his 
essay  on  the  chemical  and  geological  relations  of  the  atmos- 
phere,^ after  showing  that  the  atmosphere  could  never  have 
contained  the  thousandth  part  of  the  vast  stores  of  carbon 
which  have  been  drawn  from  it,  proposes  the  theory  that  the 
atmosphere  of  our  earth  is  essentially  a  local  condensation 
of  the  gases  which  are,  in  a  very  attenuated  form,  distributed 
through  the  realms  of  space.  From  this  vast  outer  realm 
the  carbonic  acid  enters  the  atmosphere  by  a  process  of  diffu- 
sion, thereby  maintaining  an  equal  supply  of  the  gas  which 
is  the  source  of  all  organic  life.  This  combination  of  carbon 
and  oxygen  being  broken  up  by  the  action  of    organic  life, 

*  "  Mineral  Physiology  and  Physiography,"  p.  30  et  seq.,  1886. 


2IO  ASPECTS   OF  THE  EARTH. 

the  latter  substance  is  set  over  to  play  its  essential  part  in 
the  support  of  animal  life  and  in  the  chemical  work  of  the 
inorganic  world.  Thus,  as  was  suggested  by  Dr.  Henry 
Wurz  in  1869,  the  plants  may  be  the  agents  by  which  the 
free  oxygen  is  returned  to  the  atmosphere  after  it  has  been 
imprisoned  in  the  union  with  carbon.  If  this  hypothesis  be 
true,  we  would  then  have  the  following  beautifully  ordered 
series  of  actions  :  The  celestial  spaces,  furnishing  us  the  car- 
bonic acid,  afford  at  the  same  time  solar  force  in  the  form  of 
heat  and  light  ;  the  plants,  making  use  of  this  force  in  their 
vital  processes,  break  up  the  combination  of  carbon  and  oxy- 
gen, and  so,  not  only  supply  themselves  with  material  neces- 
sary for  their  sustenance,  but  preserve  the  balance  in  the 
amount  of  oxygen  without  which  animal  life  cannot  be  main- 
tained. 

We  cannot  yet  consider  it  proved  that  this  balance  of 
carbon  and  oxygen  is  preserved  by  the  incoming  of  the  com- 
bined material  from  the  realms  of  space.  There  are,  indeed, 
some  difficulties  to  be  explained  before  the  hypothesis  can  be 
regarded  as  verified  ;  yet  it  is  by  far  the  most  satisfactory 
view  which  has  been  suggested  as  to  the  source  of  these 
aerial  springs  of  life,  which,  though  always  drawn  upon,  seem 
never  to  run  dry.  There  is,  indeed,  a  fascination  in  the  idea 
that  our  fuel,  our  daily  bread,  even  the  breath  of  life  itself, 
as  well  as  all  force  which  is  embodied  in  living  beings,  is 
constantly  and  regularly  fed  into  us  from  these  grim  and 
seemingly  inhospitable  realms  of  space. 

There  is  much  support  to  be  found  for  the  foregoing 
hypothesis  as  to  the  source  of  carbonic  acid,  in  the  evident 
uniformity  in  the  supply  of  both  carbon  and  oxygen  which 
has  been  given  to  our  atmosphere  from  the  earliest  geologi- 


THE  IXSTABILITV  OF  THE  ATMOSPHERE.  21  i 

cal  times.  Nothing  could  have  so  well  maintained  unifor- 
mity  In  the  supply  of  these  substances  as  the  constant  con- 
densation of  the  materials  from  the  spaces  between  the  stars. 
If  the  restoration  came  through  any  such  paroxysmal  actions 
as  are  involved  In  volcanic  explosions.  It  might  well  have 
happened  that  the  variations  In  that  amount  contributed  to 
the  atmosphere  would  have  been  so  great  as  to  shock  the 
delicate  mechanism  of  plant  and  animal  life. 

It  should  not  be  overlooked  that  this  hypothesis  as  to 
the  supply  of  carbon  and  oxygen  from  celestial  spaces  to  our 
atmosphere  presents  certain  grave  dlf^cultles  which  have  not 
yet  been  met.  All  our  observations  concerning  the  nature  of 
the  ethereal  substance  which  occupies  the  interstellar  spaces 
through  which  the  suns  and  planets  move,  go  to  show  that 
there  is  very  little  material  having  the  properties  of  ordinary 
gas  within  this  vast  realm.  Through  space  the  planets  evi- 
dently move  encountering  practically  no  resistance.  It  is 
therefore  difTficult  to  conceive  how  there  can  be  any  consider- 
able amount  of  gaseous  matter  In  the  regions  of  space  occu- 
pied by  our  solar  system,  for  if  such  matter  existed  there,  It 
should  manifest  Its  presence  In  the  resistance  It  offered  to  the 
movement  of  the  planetary  spheres  as  well  as  of  the  comets 
which,  owing  to  their  small  bulk,  should  show  the  effect  of 
any  materials  even  if  they  were  present  in  an  extremely  dif- 
fused form.  It  seems  to  me  possible  that  the  carbon  may 
come  to  the  earth's  atmosphere  In  the  form  of  small  meteor- 
ites containing  carbon,  which,  being  raised  to  a  high  temper- 
ature by  the  friction  which  they  encounter  in  their  passage 
through  the  air,  are  completely  burned  in  the  upper  levels  of 
the  atmospheric  envelope.  We  know  that  a  good  deal  of  car- 
bon Is  contained  in  many  meteorites,  and  that  at  certain  times 


212  ASPECTS    OF  THE  EARTH. 

of  the  year  vast  numbers  of   these  bodies  enter   the  air  and 
are  burned  up  before  they  attain  the  surface. 

We  have  now  considered  the  stabihty  of  the  air  in  its 
larger  aspects  ;  we  have  seen  that  it  has  probably  remained 
substantially  unchanged  from  an  inconceivable  period  in  the 
past.  We  may  safely  term  this  period  a  hundred  million 
years  ;  though  as  such  a  duration  is  quite  inconceivable  by 
the  human  mind,  we  do  not  help  our  statement  by  putting  it 
in  this  form.  Let  us  now  turn  to  the  more  familiar  phenom- 
ena connected  with  the  atmospheric  movements  which  we 
term  winds. 

Both  the  aqueous  and  the  aerial  envelopes  of  the  earth's 
surface  have  a  complicated  system  of  circulation.  In  the 
water-envelope  this  circulation  is  accomplished  in  two  ways. 
Within  the  sea  there  are  extensive  movements — those  of  the 
various  classes  of  ocean-currents,  which  are  mostly  the  pro- 
duct, directly  or  indirectly,  of  the  atmospheric  movements. 
When  in  the  state  of  vapor,  the  water,  borne  about  by  the 
winds,  circulates  through  the  air  until  it  finds  its  way  back 
upon  the  surface  in  the  form  of  rain,  snow,  or  dew.  These 
principal  movements  are  brought  about  by  the  action  of  the 
sun's  heat.  A  considerable  part  of  the  atmosphere  is  always 
contained  in  the  water  in  what  we  may  term  a  dissolved  form, 
and  so  makes  its  way  in  the  rain,  in  the  rivers,  and  in  the 
motions  of  the  sea. 

Although  the  winds  are  the  most  familiar  to  us  of  any  of 
the  larger  phenomenal  movements  which  take  place  upon  the 
earth's  surface,  it  was  long  before  men  came  to  anything  like 
a  clear  understanding  of  the  causes  which  produce  them.  It 
was  not,  indeed,  until  the  barometer  was  invented,  and  until 
that  instrument  came   into  common   use,  that   it  was  possible 


THE  lASTABlLITF  OF  THE  ATMOSPHERE.  213 

to  begin  a  study  of  the  causes  which  affect  the  motion  of 
the  winds.  Although  this  instrument  was  given  to  us  by 
the  illustrious  Torricelli  in  the  seventeenth,  it  was  not  until 
about  the  beginning  of  the  present  century  that  the  obser- 
vations with  it  became  sufficiently  extended  to  afford  a  fair 
clew  to  the  nature  of  the  atmospheric  movements.  Even 
in  the  present  day  a  considerable  number  of  the  problems 
which  we  encounter  in  the  study  of  the  winds  remain  un- 
solved ;  still  the  general  laws  which  induce  their  movements 
are  fairly  well  known,  and  it  is  possible  to  give  the  reader 
a  clew  to  the  more  important  facts  concerning  atmospheric 
currents.  It  should,  however,  be  understood  that  the  state- 
ments concerning  the  winds  which  can  be  made  within  the 
limits  of  this  essay  are  necessarily  brief,  and  cannot  afford  the 
reader  more  than  the  most  general  idea  regarding  the  nature 
of  these  movements.  It  is  not  in  our  project  to  consider  the 
physiology  of  winds,  but  only  to  view  them  as  phenomena 
which  affect  our  general  conception  of  the  atmospheric  work. 

We  note  at  the  outset  that  the  winds  are  in  a  general  way 
divisible  into  two  groups — those  which  we  may  describe  as 
continuous,  and  those  which  we  may  term  variable.  Though 
the  line  of  separation  between  these  groups  is,  as  might  be 
expected,  obscure,  it  has  a  considerable  value.  The  contin- 
uous movements  of  the  atmosphere  are  represented  by  the 
familiar  trade-winds  which  exist  in  certain  parts  of  the  open 
seas  north  and  south  of  the  equator.  There  alone,  on  the 
surface  of  the  earth,  do  these  movements  of  the  air  have  the 
permanence  which  we  find  associated  with  the  larger  opera- 
tions of  nature.  The  permanent  winds  of  the  upper  atmos- 
phere are  probably  more  continuous  and  more  extensive  than 
those  which  are  found  upon  the  surface  ;  but   owing  to  their 


2  14  ASPECTS   OF  THE  EARTH. 

heio-ht,  and  therefore  to  the  difficulties  of  observing  them, 
their  directions  and  velocities  are  not  so  well  known  as  the 
less  enduring  currents  which  affect  the  very  surface  of  the 
earth.  We  can  best  illustrate  the  nature  of  the  trade-winds 
by  an  imaginary  journey  from  high  altitudes  toward  the  equa- 
tor. A  voyage  such  as  is  taken  by  every  ship  from  Brit- 
ish ports,  or  from  those  of  New  England,  on  its  way  around 
Cape  Horn,  or  the  Cape  of  Good  Hope,  gives  the  observer  an 
opportunity  to  study  these  winds.  At  the  outset  of  such  a 
cruise  the  mariners  find  themselves  in  a  region  where  the  wind 
"  bloweth  as  it  listeth,"  the  uncertainty  of  the  direction  being 
the  only  foreseeable  feature  of  the  movement.  There  is  in 
these  winds  a  certain  predominance  of  a  movement  to  the 
east,  which  the  mariner  takes  into  account  ;  but  in  the  great 
atmospheric  churn  of  the  Northern  Atlantic  all  the  laws  of 
wind-movement  are  concealed  by  the  contentions  between 
the  diverse  atmospheric  influences  which  exist  there. 

As  the  ship  works  to  the  southward  into  the  open  sea, 
and  comes  near  to  the  thirtieth  parallel  of  north  latitude, 
we  find  that  the  variable  winds  gradually  die  away,  giving 
place,  after  a  brief  interval  of  calms,  to  a  constant  breeze  from 
the  east  and  north  points  of  the  compass.  At  first  these  winds 
blow  in  a  faltering  way  ;  but  shortly  they  increase  in  steadi- 
ness, and  in  the  speed  at  which  they  move,  until  the  whole  air 
flows  toward  the  southwest.  This  steadfastness  of  movement 
is  maintained  over  the  zone  which  occupies  all  the  space  of  the 
sea  except  a  relatively  narrow  belt  near  either  shore.  Very 
rarely  do  wandering  disturbances  mar  the  uniformity  of  this 
aerial  tide,  and,  at  most,  they  cause  only  a  temporar)^  break  in 
the  otherwise  continuous  movement.  After  passing  through 
this  belt  of  gentle  easterly  winds  for  a  north  and  south  dis- 


THE  IXSTABILITY  OF  THE  ATMOSPHERE.  215 

tance  of  about  thirteen  hundred  miles,  or  to  within  two  or 
three  hundred  miles  of  the  equator,  we  find  ourselves  grad- 
ually entering  a  belt  of  calms,  generally  about  three  hundred 
miles  in  width.  Through  this  region  the  sails  are  filled  by  the 
most  fitful  winds  of  the  seas,  severe  thunder-storms  with  fierce 
squalls'  alternating  with  long  periods  when  there  is  scarcely 
any  movement  in  the  air.  Availing  himself  of  the  perplex- 
ing accidents  of  the  atmosphere,  the  mariner  works  his  way 
through  this  disturbed  region  of  alternating  tempests  and 
calms  until  he  strikes  the  southern  trades,  the  exact  counter- 
part of  the  winds  of  the  north.  These  southern  trades  blow 
from  the  southeast,  as  those  from  the  north  of  the  equator 
from  the  northeast.  The  belt  of  southern  trades  has  about 
the  same  width  as  that  traversed  in  the  north.  Passino- 
through  it,  the  ship  encounters  again  in  the  Southern  Atlantic 
region  a  district  of  partial  calms  about  the  tropic,  south  of 
which  it  again  enters  upon  a  region  of  variable  winds. 

A  north  and  south  journey  in  the  Pacific  shows  us  the  same 
arrangement  of  the  permanent  and  impermanent  winds  which 
we  find  in  the  Atlantic.  Though  the  energy  of  these  winds 
is  not  the  same  as  that  of  those  in  the  Atlantic,  they  have 
an  even  greater  steadfastness.  This  marvellous  regularity  of 
their  movements  was  a  delightful  surprise  to  the  early  naviga- 
tors. Varenius,  exaggerating  the  truth  somewhat,  declares 
that  on  arriving  at  Acapulco,  on  the  west  coast  of  South 
America,  the  helm  of  the  ship  might  be  lashed  and  tiie  sailors 
go  to  sleep,  and  they  might  still  make  their  port  in  the  Philip- 
pines, on  the  western  side  of  that  ocean.  "The  Spaniards 
called  the  trade-wind  region  '  El  golfo  de  las  damas,'  for  when 
once  it  was  reached  a  girl  might  take  the  helm."  " 

*  R.  H.  Scott  :  "  Elementary  Meteorology,"  p.  244.     London,  1885. 


2i6  ASPECTS  OF  THE  EARTH. 

It  is  evident  that  this  distribution  of  the  aerial  currents  is 
a  permanent  feature  on  the  surface  of  the  globe.  The  earliest 
navigators  of  the  oceans  found  the  constant  and  the  variable 
areas  exactly  where  we  find  them  to-day.  The  ships  of  Colum- 
bus were  borne  westward  by  the  northern  belt  of  trades,  and 
every  sailor  who  since  that  day  has  traversed  the  field  has 
availed  himself  of  their  movement.  These  gentle  breezes  are 
among  the  most  steadfast  features  of  the  earth  ;  they  are  older 
than  the  continents  ;  they  have  indeed  endured  from  the  time 
when  our  geological  records  began  to  be  written  in  the  rocks. 
The  primal  cause  of  these  constant  winds,  as  well  as  of  all  the 
atmospheric  movements  of  importance,  is  to  be  found  in  the 
unequal  distribution  of  the  sun's  heat  upon  the  earth's  surface. 
If  the  earth  presented,  as  men  first  imagined  it  did,  a  plane 
surface  to  the  sun,  there  would  be  no  such  system  of  constant 
winds  as  we  have  indicated,  for  the  reason  that  the  heat  would 
be  equally  distributed,  and  there  would  thus  be  a  want  of  the 
disturbing  causes  which  set  the  air  into  these  more  ordered 
movements.  But  the  spherical  shape  of  the  earth  causes  the 
sun's  heat  to  fall  in  very  different  share  on  the  equatorial  region 
and  on  the  districts  about  the  poles.  Within  the  tropics,  where 
the  sun  is  from  time  to  time  vertical,  and  at  most  departs  but 
slightly  from  that  position  during  the  course  of  the  year,  far 
more  heat  falls  upon  the  earth  than  comes  to  the  surface 
within  the  polar  circles.  This  greater  amount  of  heat  received 
within  the  tropical  belt  of  land  and  sea  warms  by  radiation  the 
layers  of  atmosphere  near  the  surface  of  the  earth  ;  the  heated 
air  expands,  and  is  lightened  by  its  expansion  to  a  greater  de- 
gree than  is  the  air  of  regions  nearer  the  poles.  It  was  at  first 
thought  that  this  heat  directly  produced  an  up-draught  from 
the  tropical  regions,  and  that  the  air  which  becomes  the  trade- 


THE  INSTABILITY  OF   THE  ATMOSPHERE.  217 

winds  flowed  in  from  the  north  and  south  to  fill  the  partial 
vacuum.  Although  this  direct  method  of  operating-  may  in  a 
measure  account  for  the  rush  of  the  trade-winds  toward  the 
equator,  it  is  by  no  means  a  sufficient  explanation  of  the  phe- 
nomenon. 

We  can  best  get  a  clear  idea  of  the  machinery  of  the  winds 
by  a  simple  illustration.  Let  us  conceive  a  tall  chimney,  such 
as  is  frequently  erected  about  manufacturing  establishments 
where  it  is  desired  to  produce  a  strong  up-draught.  For  con- 
venience, let  us  imagine  that  this  chimney  is  closed  at  the  top 
when  we  begin  to  heat  a  column  of  air  within  it  which  pre- 
viously was  at  the  temperature  of  the  surrounding  atmosphere. 
As  soon  as  we  have  applied  heat  at  the  base  of  the  column,  it 
is  evident  that  the  air  tends  to  rush  upward  in  the  shaft  and 
brings  an  increase  of  pressure  upon  the  summit.  This  pres- 
sure is  due  to  the  fact  that  the  external  air  between  the  chim- 
ney-top and  its  base  weighs  more  than  the  air  within  the 
chimney  in  its  heated  state.  If  now  we  remove  the  cap  from 
the  chimney,  the  air  within  this  shaft  will  escape  from  the 
top  ;  and  if  there  be  no  wind,  will  flow  off  on  every  side  over 
the  surface  of  the  colder  air.  Another  familiar  illustration 
may  aid  the  reader  to  clear  his  mind  as  to  the  nature  of  this 
action.  Let  him  imagine  a  trough-shaped  vessel  divided  into 
three  compartments,  those  at  either  end  filled  with  water  and 
the  central  space  with  oil,  which,  as  he  will  remember,  is 
slightly  lighter  than  the  water.  If  now  we  remove  the  bar- 
riers which  separate  the  oil  from  the  water,  on  either  side, 
we  shall  see,  as  the  eye  clearly  notes,  that  the  water  slips 
under  the  oil  and  the  oil  over  the  water.  It  is  not  neces- 
sary to  try  the  experiment  in  order  that  it  may  be  well  con- 
ceived in  that  laboratory,  the  mind's  eye.     We  have  now  only 


2i8  ASPECTS   OF  THE  EARTH. 

to  suppose  that  by  some  process  the  oil  should  become  water 
as  it  flowed  toward  either  end  of  the  vessel,  and  the  water  to 
become  oil  as  it  approached  the  central  part,  to  construct  a 
convenient  image  of  the  process  by  which  the  air  rises  over 
the  equatorial  belt,  and  so  leads  to  a  current  toward  the  equa- 
tor, along-  the  surface  of  the  earth,  and  toward  the  poles  in  the 
higher  atmosphere.  Assuming  that  the  reader  now  conceives 
how  this  primal  difference  in  heat  brings  about  the  movement 
from  high  altitudes  to  low,  along  the  earth's  surface,  and  from 
low  altitudes  to  high,  in  regions  considerably  above  the  earth, 
we  may  advance  one  step  further  in  our  considerations. 

The  next  puzzling  feature  in  the  movement  of  the  per- 
manent winds  is  found  in  the  fact  that  these  currents  do  not 
move  on  north  and  south  lines,  as  we  should  at  first  sight 
expect  them  to  do,  but  the  southward-moving  winds,  or  those 
which  in  the  northern  hemisphere  seek  the  equator,  blow  from 
the  points  between  the  east  and  north  ;  while  the  upper  cur- 
rents, which  convey  the  air  back  from  the  equator  to  high 
altitudes,  move  in  the  rev^erse  direction,  or  from  southwest  to 
northeast.  Although,  as  before  remarked,  our  information 
concerning  this  upper  air-current  is  limited,  its  constancy, 
swiftness,  and  general  course  are  sufficiently  proved  by  obser- 
vations made  on  the  summit  of  high  mountains  within  the 
trade-wind  belt,  as  well  as  by  the  movements  of  clouds  in  the 
principal  regions  of  the  atmosphere. 

As  long  ago  as  1735  an  attempt  was  made  to  explain  the 
oriofin  of  this  deflection  of  the  winds  from  the  true  north  and 
south  course.  Although  the  explanation  does  not  give  a  full 
account  of  the  phenomenon,  it  still  retains  a  place  in  the  most 
of  our  text-books.  We  owe  this  account  of  the  trade-wind 
movement  to  George  Hadley.      His  explanation  rests  on  the 


THE  INSTABILITY  OF   THE  ATMOSPHERE.  219 

fact  that  when  a  particle  of  air  or  of  water,  or  any  other  mat- 
ter, moves  from  the  poles  toward  the  equator,  or  from  higher 
to  lower  latitudes,  it  is  constantly  proceeding  into  regions 
having  higher  rates  of  movement,  by  virtue  of  the  earth's 
rotation,  than  those  from  which  it  came,  and  so,  by  virtue  of 
its  inertia,  it  constantly  falls  away  to  the  westward.  The  earth 
in  its  rotation  slips  to  a  certain  extent  beneath  it.  In  the 
reverse  way,  a  particle  starting  from  the  equator,  where  it 
moves,  by  virtue  of  the  earth's  rotation,  at  the  rate  of  a  thou- 
sand miles  an  hour  in  an  eastward  direction,  and  proceed- 
ing toward  the  poles,  where  it  will  not  have  any  translatory 
motion  on  account  of  the  revolution  of  the  earth,  is  constantlv 
coming  into  regions  having  a  less  eastward  movement  than 
it  at  the  moment  possesses,  and  so  outruns  the  movement  of 
the  earth,  inclining  in  an  eastward  direction. 

The  reader  can  again  illustrate  this  principle  by  an  experi- 
ment, which  he  may  try  in  practice,  or  essay  in  his  imagina- 
tion, by  endeavoring  to  walk  from  the  centre  of  a  railway 
turn-table,  such  as  is  used  for  reversing  the  position  of  loco- 
motives, to  the  periphery  of  that  disk.  He  will  conceive, 
or  by  an  experiment  he  will  have  it  proved  to  him,  that  he 
cannot  walk  on  a  straight  line  from  the  centre  to  the  cir- 
cumference when  the  disk  is  turning,  but  will  attain  a  point 
on  the  periphery  behind  the  point  at  which  a  radius  of  the 
circle  intersects  that  line.  Standing  a  moment  on  the  peri- 
phery, so  that  his  body  may  acquire  the  rotative  movement 
of  the  disk,  he  will  see  that  in  walking  toward  the  centre 
he  again  inclines  to  one  side,  because  the  momentum  of  his 
body  makes  it  dif^cult  for  him  to  acquire  the  movement  of 
the  surface  to  which  his  successive  steps  bring  him.  When, 
however,  we  endeavor  to  apply  the  truth  which   Hadley  dis- 


2  20  ASPECTS   OF  THE  EARTH. 

covered  to  the  spherical  surface  of  the  earth,  we  find  it 
insufficient  to  account  for  the  deflection  of  moving  bodies 
on  that  surface.  Pendulum  experiments  of  the  distinguished 
Foucault,  made  in  the  middle  of  this  century,  showed  that, 
while  Hadley's  considerations  were  true,  another  principle  is 
involved  in  the  movement  of  the  winds  and  of  the  ocean- 
currents.  This  principle  is  that,  owing  to  the  fact  that  the 
earth  rotates  from  west  to  east,  all  bodies  moving  freely  upon 
its  surface  will  deflect  on  one  direction,  the  measure  of  the 
deflection  being  due  to  the  latitude  of  the  point  and  the 
velocity  of  the  moving  particles.  It  is  so  difficult  to  give  a 
popular  explanation  of  this  principle,  and  its  comprehension 
is  so  far  unnecessary  to  our  aim  here,  that  we  may  fairly  ask 
the  reader  to  accept  this  statement,  or  to  look  elsewhere  for 
a  detailed  explanation. 

It  is  worth  the  reader's  while  to  conceive,  as  well  as  he 
may,  the  general  principles  which  control  the  movements  of 
the  constant  winds,  for  upon  these  movements,  in  a  great 
measure,  depends  the  whole  system  by  which  heat  is  distrib- 
uted over  the  surface  of  the  earth.  This  distribution  is  one 
of  the  many  conditions  on  which  the  habitability  of  the 
globe  absolutely  depends.  If  the  heat  which  comes  upon  the 
earth's  surface  from  the  sun  stayed  where  it  fell,  if  there  were 
no  machinery  compensating  for  the  irregularities  arising  from 
the  excessive  supply  which  falls  in  the  tropics  and  the  scant 
measure  given  to  high  latitudes,  the  equatorial  region  would 
be  too  hot  for  life,  and  the  regions  beyond  the  parallels  of 
forty  degrees  north  and  south  of  the  equator  would  be  too 
cold  ;  they  would  be  locked  in  eternal  frost.  This  compen- 
sation, it  is  true,  is  only  in  a  small  measure  effected  by  the 
winds  themselves ;    for,   although    they  represent   the    move- 


THE  INSTABILITY  OF   THE  ATMOSPHERE.  22  1 

ment  of  a  great  body  of  air  to  and  from  the  equatorial  belt, 
this  air  has  very  little  heat-storing  power,  owing  to  its  gaseous 
elements.  The  work  of  compensation  is  accomplished  in  the 
main  by  the  ocean-currents  which  the  winds  induce.  The 
trade-winds,  moving  the  surface-waters  over  which  they  rub, 
drive  along  a  broad  sheet  of  the  ocean's  surface  from  either 
atmosphere  toward  the  equator.  If  these  winds  moved 
squarely  down  upon  the  equator,  the  result  would  be  that 
the  waters  would  soon  be  heaped  up  under  that  line,  and  the 
currents  of  the  water  would  cease  to  flow ;  but  as  they  move 
obliquely  from  the  northeast  and  from  the  southeast  toward 
the  equatorial  belt,  they  produce  at  their  junction  a  wide 
westerly-setting  current  which  flows  at  the  rate  of  two  or 
three  miles  an  hour.  When  this  current  comes  against  the 
shoals  of  a  continent,  as  it  does  against  South  America,  it 
divides  and  turns  in  two  streams  toward  either  pole.  In 
the  case  of  the  Gulf  Stream  the  great  equatorial  tide  sweeps 
on  toward  the  northern  seas,  bearincr  with  it  a  oreat  store  of 
tropical  heat.  To  it  Europe  owes  its  habitability,  and  the 
region  within  the  arctic  circle  receives  from  it  more  heat, 
as  Dr.  James  Croll  has  shown,  than  comes  to  it  from  the 
direct  rays  of  the  sun.  We  see  by  this  instance,  one  of  many 
which  could  be  adduced,  that  the  atmosphere  not  only  gives 
the  primal  conditions  of  life,  but  by  its  great  movements 
secures  to  the  larger  part  of  the  land  and  sea  temperatures 
suited  to  the  existence  of  that  life. 

Not  only  do  we  owe  to  the  ocean  streams  the  present 
fitness  of  the  greater  land  areas  for  the  uses  of  organic  life, 
but  during  periods  in  the  past  history  of  the  earth  of  which 
we  have  no  record  written  in  the  rocks,  durine  a  time  which 
certainly  cannot  be  reckoned  at  less  than   100,000,000  years, 


2  22  ASPECTS   OF  THE  EARTH. 

the  sea-currents  have  been  doing  this  same  beneficent  work 
of  taking  excessive  heat  from  tropical  regions  to  warm  the 
fields  of  sea  and  land,  which,  owing  to  their  high  latitudes, 
would  otherwise  have  an  extremely  low  temperature.  The 
trade-winds  and  the  ocean-currents,  which  in  good  part  if  not 
altogether  are  due  to  these  great  movements  of  the  air,  are 
among  the  most  permanent  features  in  the  physical  machinery 
of  our  earth.  The  trade-winds  have  doubtless  blown,  and  the 
ocean-currents  streamed  away  through  the  seas,  since  the 
dawn  of  geological  history.  From  the  mOst  ancient  times 
lands  have  existed  which  served  to  divert  the  streams  of 
ocean  water  from  the  tropical  belt  to  high  latitudes.  Thus, 
in  the  early  history  of  North  America,  we  can,  from  the  evi- 
dence of  coral  reefs,  perceive  that  in  the  lower  Silurian,  upper 
Silurian,  and  Devonian  ages  the  vast  tide  of  what  we  now 
term  the  Gulf  Stream  flowed  up  through  a  great  arm  of  the 
sea  which  long  occupied  the  Mississippi  valley,  and  made  its 
way  by  that  passage  toward  the  arctic  circle,  bringing  a  tide 
of  warmth  and  life  to  the  high  latitudes  of  the  northern  hemi- 
sphere. Though  much  changed  in  position,  diminished  per- 
haps at  times  in  volume,  perhaps  at  times  altogether  diverted 
from  the  northern  hemisphere,  this  stream  has  flowed  on 
through  all  the  seons  of  creoloo^ical  time. 

We  now  turn  to  the  second  great  group  of  atmospheric 
currents,  those  which  constitute  the  inconstant  winds.  This 
group  of  air-currents  affords  a  larger  and  more  puzzling  class 
of  movements,  more  puzzling  because  they  depend  upon  the 
interaction  of  many  variable  conditions.  As  to  them  all  we 
may  make  the  same  general  statement  which  we  have  already 
made  ccicerning  the  constant  winds,  viz.  :  That  they  are 
primarily   due    to    the   excess    of    temperature    in    the    lower 


regions  of  the  atmosphere,  caused  by  the  fact  that  the  incur- 
rent  heat  from  the  sun  passes  more  readily  through  the  air 
than  the  radiant  heat  does.  Starting  from  this  general  prin- 
ciple, we  find  that  the  inconstant  winds  fall  naturally  into 
two  categories  :  First,  those  which  are  caused  by  the  differ- 
ence in  the  condition  of  the  air  over  the  land  and  over  the 
sea  ;  second,  disturbances  which  are  due  to  a  violent  move- 
ment of  the  heated  air  which  lies  upon  the  earth's  surface 
to  escape  into  the  upper  regions  of  the  atmosphere,  where- 
unto  its  lightness,  due  to  the  heat  it  has  acquired  from  the 
surface,  makes  it  tend.  The  first  of  these  two  groups  of 
inconstant  winds  affords  us  the  class  of  what  are  commonly 
termed  land-  and  sea-breezes,  the  effects  of  which,  though 
interesting,  are  of  relatively  small  importance  in  the  economy 
of  the  world. 

The  simplest  case  arising  from  the  difference  in  the  con- 
dition of  the  air  over  land  and  ocean  may  be  noted  where  a 
considerable  island  rises  from  a  space  of  tropical  open  seas. 
A  brief  experience  on  such  an  island  shows  us  that  in  the 
afternoon  of  each  day  a  wind  sets  in  from  the  sea  and  dies 
away  about  sunset.  For  a  while  the  air  is  still,  but  toward 
midnieht  a  steadfast  current  sets  in  from  the  other  direction, 
namelv,  from  the  land,  and  blows  until  after  sunrise.  Thus 
the  normal  atmospheric  conditions  of  the  island  give  us  alter- 
nating breezes  enduring  for  about  equal  times,  but  moving  in 
opposite  directions.  Here  again  we  have  to  correct  the  usual 
statement  as  to  the  origin  of  the^e  winds.  It  is  generally 
said  that  the  air,  becoming  heated  over  the  surface  of  the 
land  as  that  surface  gains  in  temperature  toward  noonday, 
rises  and  so  draws  in  the  air  from  the  sea,  while  at  night  the 
reverse  action  takes  place.     This  theory  is  disproved  by  the 


2  24  ASPECTS    OF  THE  EARTH. 

circumstance  pointed  out  two  centuries  ago  by  Dampier,  that 
the  sea-breeze  begins  in  the  offing  and  extends  gradually  to 
the  coast,  while  the  land-breeze  comes  off  from  the  shore 
and  forces  its  way  out  to  sea.  Dampier's  statements  about 
the  sea-breeze  are  :  "  It  comes  in  an  even,  small  black  curl 
upon  the  water,  whereas  all  the  sea  between  it  and  the  shore 
not  reached  by  it  is  smooth  and  even  as  glass  in  comparison. 
In  an  hour's  time  after  it  reaches  the  shore  it  fans  pretty 
briskly,  and  so  increases  gradually  until  twelve  o'clock  ;  then 
it  is  commonly  strongest  and  lasts  until  two  or  three,  a  very 
brisk  gale!"*  Although  the  difference  in  temperature  in 
the  surfaces  of  the  land  and  sea  is  the  important  cause  of 
these  changing  currents,  the  method  of  action  is  probably 
not  that  just  stated,  but  comes  about  as  follows :  The  air 
from  the  surface  of  the  land,  being  expanded  by  heat,  is 
raised  more  or  less  above  the  surface,  so  that  the  levels  of 
equal  barometric  pressure  are  higher  over  the  island  than  they 
are  over  the  sea,  as  is  indicated  in  the  diagram.  This  dif- 
ference in  elevation  of  the  levels  of  equal  barometric  pressure 
causes  the  air  to  slide  off  from  over  the  surface  of  the  island 
to  the  portion  of  the  atmosphere  above  the  surface  of  the 
sea,  thus  increasing  the  pressure  at  the  last-named  points. 
This  pressure  directly  forces  the  sea  air  in  toward  the  island. 
Gradually,  after  the  sun  goes  down,  the  land-surface  cools 
until  its  temperature  is  below  that  of  the  sea,  when  the  fore- 
going process  is  reversed.  The  lines  representing  equal 
barometric  pressure  over  the  land  come  nearer  together  ;  the 
air  then  flows  in  from  the  upper  regions  of  the  ocean  atmos- 
phere, weights  the  column  of  air,  and  forces  the  current  out 
along  the  surface  to  the  seaward. 

*  R.  H.  Scott  :  "  Elementary  Meteorology,"  p.  286. 


THE  INSTABILITV  OF   THE  ATMOSPHERE.  225 

Along  the  margin  of  the  continents  we  frequently  find  in- 
dications of  land-  and  sea-breezes,  which,  although  much  more 
perturbed  than  in  the  case  of  oceanic  islands,  are  still  clearly 
due  to  the  operation  of  the  same  forces.  The  east  wind 
which,  in  the  season  of  hot  but  still-aired  summer  days,  creeps 


Land-  and  Sea-Breezes,  No.  i.      Currents  of  Air  by  Day. 
[In  this  diagram,  as  in  No.  2,  the  dotted  lines  represent  like  temperatures.] 

in  upon  the  shore  of  New  England  and  other  parts  of  this 
continent,  is  an  instance  of  this  action.  In  the  months  of 
May  and  June  the  sea-water  off  the  New  England  coast  is 
often  as  much  as  thirty  or  forty  degrees  cooler  than  the 
surface  of  the  land,  and  the  air  over  these  surfaces  for  a 
considerable  height  above  the  sea  differs  nearly  as  much  in 
its  temperature.  Whenever  there  is  no  wind  from  the  con- 
tinent   this    air    from   the  sea  flows  in  beneath    that   on    the 


Land-  and  Sea-Breezes,   No.  2.      Currents  of  Air  by  Night. 

land,  sometimes  with  considerable  speed.  It  is  interesting  to 
watch  the  process  of  this  movement,  as  it  may  frequently  be 
observed  along  these  shores,  for  it  is  the  type  of  many  of  the 
aerial  movements  which  are  not  so  observable.  Selectino-  a 
still  summer  day,  and  a  point  on  the  shore  at  the  sea-level. 


»  -.■5S'V<»--4#^  ■  •■  ■  ""•  ■"'- 


226  ASPECTS    OF  THE  EARTH. 

we  may  await  the  coming  of  the  aerial  tide.  It  approaches 
the  shore  in  the  form  of  a  wedge,  which  sHps  under  the 
heated  air  of  the  land.  At  first  the  thin  point  of  this  wedge 
may  be  only  a  foot  or  two  deep,  and  has  only  a  very  slight 
motion,  as  may  be  shown  by  the  smoke  of  burning  paper,  or 
even  by  the  effect  of  temperature  on  the  hand  when  it  is  held 
near  the  ground.  The  cold  air  gradually  becomes  deeper,, 
but  for  an  hour  it  may,  in  some  cases,  not  be  fifty  feet  in 
depth  ;  so  that  on  the  lower  floor  of  a  tall  house  we  may  find 
the  cool  air  creeping  in  from  the  sea,  and  on  the  upper  story 

we  may  note  a  re- 
verse movement  of 
the  warm  air  from 
the  land  seaward. 

We  have  now 
considered  those 
m.ovements  of  the 
air  which  are  more 
or  less  constant  or 

A    Whirlwind. 

regular  in  their  ac- 
tion. We  therefore  turn  to  the  group  of  irregularly  variable 
winds.  It  is  characteristic  of  these  winds  that  they  are  tempor- 
ary in  their  nature,  often  very  violent,  therefore  not  to  be  pre- 
dicted, as  are  the  constant  movements  of  the  atmosphere.  Like 
the  preceding  class,  they  are  due  to  differences  of  temperature 
of  the  air  upon  the  surface,  and  in  higher  levels  of  the  atmos- 
phere brought  about  by  the  action  of  solar  heat.  They  may, 
for  convenience,  be  divided  into  three  distinct  groups,  which 
receive,  respectively,  the  names  of  whirlwinds,  tornadoes, 
and  cyclones.  All  three  of  these  classes  of  inconstant  winds 
are  found  both  on  sea  and   on  land,  but  the  two   latter  are 


Wis 


THE  INSTABILITY  OF  THE  ATMOSPHERE.  227 

much  more  common  on  the  land-surfaces,  or  on  the  portions 
of  the  ocean  near  the  shore,  than  in  the  open  sea.  All  these 
groups  of  winds  have  certain  common  characteristics  which 
indicate  a  likeness  in  the  circumstances  of  their  origin.  They 
all  exhibit  a  more  or  less  distinct  spiral  motion  in  the  air 
involved  in  their  movements  ;  they  all  show  a  distinct  ascend- 
ing movement  of  the  air  in  their  central  parts.  In  all  of 
them  this  central  part,  the  shaft  of  the  whirl,  has  a  more  or 
less  forward  motion,  and  in  the  larger  whirls  the  direction  of 
this  motion  is  tolerably  regular  in  each  region  where  they 
occur. 

The  common  cause  of  this  whirling  movement  is  the  exist- 
ence of  a  heated  layer  of  air  next  the  surface  of  the  earth, 
which  air,  by  virtue  of  its  greater  heat,  tends  to  be  more 
expanded,  and  therefore  lighter,  than  the  overlying  cooler 
mass  of  atmosphere.  With  certain  trifling  exceptions,  to  be 
noted  further  on,  the  heat  of  this  sheet  of  air  next  the  sur- 
face of  the  earth  is  due  to  the  fact  that  the  direct  rays  of  the 
sun  pass  more  easily  through  the  atmosphere  than  do  those 
of  the  rebounding  or  radiant  heat  which  flows  from  the 
earth's  surface  outward  into  space.  The  result  is  that  the 
ground,  becoming  more  heated  than  the  overlying  air,  gives 
out  its  heat  to  the  layer  of  the  atmosphere  just  above  its 
level,  and  so  creates  a  heated  stratum  which,  on  account  of 
its  gain  in  temperature,  seeks  to  find  a  way  upward.  For  a 
time,  if  there  be  no  wind,  this  buoyant  air  may  be  shut  in  by 
the  layer  of  cooler  air  which  overlies  it,  and  through  which  it 
finds  no  open  path  ;  but  as  the  sheet  grows  thicker  it  finally, 
by  some  chance,  makes  a  way  through  the  stratum  which 
holds  it  down  and  escapes  to  the  upper  regions  of  the  atmos- 
phere, to  which  its  buoyancy  impels  it.     A  little  experiment 


2  28  ASPEC7S   OF  THE  EARTH. 

will  show  the  essential  principles  of  this  movement  in  sub- 
stances which  are  more  visible  than  these  sheets  of  air,  and 
on  a  scale  more  readily  comprehensible.  Placing  a  layer  of 
oil  on  the  surface  of  a  flat  vessel,  it  is  possible,  with  great 
care,  to  float  a  sheet  of  water  over  it  so  that  the  superim- 
posed water  is  of  considerable  thickness.  We  now  have  a 
lio-hter  fluid  below  and  a  heavier  above.  This  is  an  unstable 
condition,  which  naturally  ends  in  upsetting  the  two  fluids— 
a  restoration  of  stability.  As  long  as  the  overlying  water  is 
perfectly  still,  the  tendency  of  the  oil  to  rise  may  not  cause 
any  movement;  but  the  slightest  disturbance  will  determine 
the  oil  to  break  through  the  overlying  water.  If  we  pass  a 
straw  through  the  water  and  make  a  little  stir  in  the  two 
fluids,  at  once  through  the  little  gap  a  stream  of  oil  sets 
upward.  From  all  sides  this  oil  slips  to  the  path  which  we 
have  formed,  and  in  a  few  seconds  the  passage  is  accom- 
plished and  a  stable  equilibrium  established.* 

With  this  experiment  in  mind,  let  us  proceed  to  examine 
any  level  surface,  on  a  hot  afternoon  when  the  air  is  very  still. 
It  is  necessary  for  the  observation  that  it  be  made  on  some 
tolerably  plain  surface  which  is  not  covered  with  vegetation, 
for  the  leaves  of  plants  radiate  the  heat  which  comes  to  them 
from  the  sun  with  great  rapidity,  and  therefore  the  surface  of 


*  This  experiment  can  be  more  readily  performed  by  choosing  some  oil  which 
becomes  partly  solid  at  a  temperature  above  the  freezing-point,  as,  for  instance, 
lard-oil.  Warming  the  oil  until  it  is  transparent,  we  pour  it  into  a  flat-bottomed 
vessel,  which  must  be  warm  enough  to  permit  tiie  oil  to  flow  freely  ;  then  placing 
the  vessel  in  another  of  cold  water,  we  permit  the  oil  to  stiffen.  Now  pour  in  the 
water,  place  the  receptacle  in  another  basin  of  water,  and  warm  gradually  to  melt 
the  oil  ;  then,  as  before,  making  a  little  stir,  we  determine  the  point  at  which  the 
oil  will  rise  through  the  superincumbent  water,  or  we  may  wait  for  some  slight  jar 
to  create  the  local  disturbance,  which  will  bring  about  the  same  result. 


THE  INSTABILITV  OF   THE  ATMOSPHERE.  229 

the  earth  beneath  them  does  not  attain  the  high' temperature 
which  we  find  it  to  have  in  reg-Ions  without  verdure.  Let  us 
note  that  the  air  next  the  surface  of  the  earth  is  vibratinQf  with 
the  heat,  so  that  if  we  stoop  down  and  look  through  the  air, 
within  a  foot  or  two  from  the  ground,  we  see  that  the  shape 
of  all  objects  dances  and  twinkles  in  the  mirage  which  is  pro- 
duced by  the  boiling  motion  which  the  radiant  heat  produces. 
With  a  thermometer  we  may  note  that  there  is  a  difference  of 
many  degrees  between  the  temperature  at  the  surface  of  the 
earth  and  at  the  height  of  a  few  feet  above  it.  The  difference 
is  so  great  that  it  often  can  be  perceived  by  holding  the  hand, 
first  at  six  inches  from  the  ground,  and  again  above  the  head. 
Beginning  at  sunrise  on  a  day  of  unbroken  calm,  this  pro- 
cess of  heatinof  the  air  next  the  grround  gfoes  on  until  after- 
noon  ;  the  tension  then  becomes  so  great  that  the  hot  air 
because  of  its  lightness  breaks  through  the  cold.  The  place 
where  the  weak  spot  in  the  overlying  layer  of  cold  air  is  found 
is  determined  by  various  accidents.  Some  heated  tree-trunk 
or  tall  object  of  any  kind,  rising  a  little  way  through  the  cold 
layer,  may  at  that  point  make  the  hot  air  thicker  than  else- 
where, and  consequently  the  strain  upward  at  this  particular 
place  will  be  greater.  As  soon  as  this  bottom  air  finds  a  way 
upward  it  swiftly  rushes  toward  the  point  of  escape,  as  is 
shown  in  the  cuts.  Immediately  after  the  up-rush  begins,  the 
air  streams  in  from  every  side  toward  the  chimney,  at  first 
slowly  ;  then,  as  it  gains  velocity,  more  and  more  swiftly.  As 
it  gets  toward  the  centre  its  velocity  is  accelerated,  and  the 
particles  of  air  crowd  against  each  other.  As  soon  as  the 
upward  movement  is  established,  we  find  that  the  particles  of 
the  atmosphere  take  on  the  whirling  movement.  It  is  not  so 
easy  to  explain  the  cause  of  this  whirling  as  it  is  to  show  the 


2  30 


ASPECTS    OF  THE  EARTH. 


Other  circumstances  of  these  centre-seeking  currents,  but  we 
can   easily  note   the   fact   that   such   movements  occur   in  all 
cases  where  a  fluid  or  a  gas  streams  rapidly  from  a  wide  field 
through  a  small  opening.      Movements  of  this  sort  can  be  seen 
in  a  bath-tub   where  there  is  a  hole  in  the  bottom    for    the 
escape  of  the  water.      Filling  the  basin  with  water  and  lifting 
the  plug,  we  see  in  a  moment  that  the  fluid  begins  to  spin 
round  as  it  flows  to  the  centre.      At  first  this  whirling  move- 
ment is  along  the  bottom  of 
the  vessel  only,  but  it  is  rap- 
idly propagated  upward  until 
for  the  whole  depth  the  water 
spins  in  the  part   next    from 
the  opening  with  such  veloc- 
ity   that    a    conical     hole    is 
formed  on  the  surface,  which 
may  extend  downward  to  the 
outlet,  and  even  for    a   little 
distance  into  the  pipe  which 
takes  the  water  away.*     Stir- 
rino-  the  water  with  a  motion 
of  the  hand,  we  can  destroy  this  whirl,  but  it  is  quickly  re- 
created.     By  giving   the  water   a  decided  movement  we  can 
reverse   the    direction   of    the   whirl,   but   in    no  way  can   we 
cause  the  water  to  escape  without  the  rotatory  motion.      We 
thus  see  that,  although  the  spiral  movement  is  essential,  the 


Diagram   of  a  Sink  Spout. 


*It  is  important  in  this  experiment  that  the  exit  opening  shall  be  unobstructed. 
In  most  cases  modern  bath-tubs  and  wash-basins  have  partitions  across  the  space, 
which  divides  the  turning  water  into  several  streams.  Each  of  these  streams  creates 
its  own  little  whirl,  but  they  react  against  each  other  in  such  a  way  that  no  con- 
siderable whirlpool  is  formed. 


THE  IXSTABIL/TF  OF  THE  ATMOSPHERE. 


2^1 


direction,  whether  to  the  right  or  to  the  left,  is  a  matter  deter- 
mined by  circumstances. 

The  cause  of  this  whirHng  movement,  as  far  as  it  can  be 
briefly  ajid  simply  stated,  is  as  follows :  When  the  particles  of 
air  or  water  begin  to  rush  toward  the  centre,  the  chance  is 
infinitely  great  that  they  will  not  all  follow  straight  lines  lead- 
ing directly  to  the  middle  of  the  column.  Now,  if  any  of  them 
fail  to  g-o  on  the  straiehtest  lines, 
they  will  have  to  curve  at  the 
end  of  their  course  in  order  to 
join  the  upward  march.  They 
thus  give  a  shove  to  one  side  of 
the  delicately  poised  column,  and 
so  set  it  spinning  round.  As 
soon  as  the  column  begins  to 
turn,  fewer  of  the  particles  can 
move  straightforwardly  to  the  \'^^^: 
centre,  and  more  press  toward 
the  side  from  which  the  column 
is  turning  and  add  their  shove  to 
the  force  which  spins  it.  When 
it  acquires  a  rapid  movement,  all  '^  oust-whiri. 

the  particles  press    on    the    same    side,  and    so   increase   the 
velocity  of  its  rotation. 

Returning  now  to  the  whirl  of  the  air — the  dust-whirl,  as 
we  shall  for  convenience  term  it — we  perceive  that  on  the  sur- 
face of  the  earth  there  is  a  broad  disk,  a  few  feet  in  depth  and, 
perhaps,  a  score  or  two  in  diameter,  through  which  the  air 
moves  toward  a  relatively  slender  vertical  shaft.  If  the  col- 
umn be  very  distinctly  developed,  and  the  dust  it  draws  up 
large  in  quantity,  we  may  be  able  to  perceive  that  at  a  few 


232 


ASPECTS    OF  THE  EARTH. 


hundred  feet  above  the  surface  the  cylinder  expands  into  a 
form  substantially  like  that  which  it  had  on  the  surface.  In 
other  words,  the  dust-whirl  has  an  hour-glass  shape,  but  the 
tube  which  connects  the  upper  and  lower  cones  is  relatively 
very  long. 

Whirlwinds  may  be  formed  by  the  heat  of  the  earth's  sur- 
face, which  is  not 
derived  from  the 
rays  of  the  sun, 
but  from  terrestrial 
sources  of  tempera- 
ture. They  are  ex- 
tremely common 
over  forest-fires, 
where  the  air  lying 
upon  a  district  of 
hundreds  of  acres 
in  extent  is  much 
heated  ;  the  heated 
air  seekinor  to  break 
through  the  cooler 
air  above,  exactly  as 
in  the  case  of  the  dust-whirl,  takes  the  form  of  a  spinning  col- 
umn. Even  in  a  large  burning  building,  careful  watching  will 
frequently  show  these  whirls  in  the  air  above  it.  In  volcanic 
eruptions  they  are  also  not  uncommon  ;  and  on  account  of  the 
intense  heat  arising  from  the  emanations  of  the  crater  they 
are  far  more  powerful  than  are  dust-whirls  or  those  developed 
by  ordinary  fires.  Some  years  ago  I  had  an  opportunity  of 
seeing  a  considerable  forest  fire  in  Bristol  County,  Mass. 
The  burning  area  sent  up  a  column  having  the  form  shown  in 


rTipg^ysif^ 


,.ciA.V^i 


'i^yfflA^^t!fe|.riUjfeg^^iyJi^^-f^^^ 


Smoke-whirl  from  Forest  Fire. 


THE  INSTABILITV  OF   THE  ATMOSPHERE.  233 

the  fiorure,  which  rose  in  the  still  air  to  the  heiofiit  of  2,000 
feet  or  more  above  the  surface.  The  engraving  is  reproduced 
from  a  sketch  made  at  the  time.  The  column  was  very  dis- 
tinct, piercing  the  still  heated  air  of  an  August  day  to  the 
height  of  a  thousand  or  more  feet  with  slight  deflection  from 
the  vertical.  Attaining  the  clear  upper  level,  the  smoke  and 
vapor  of  water  became  diffused,  forming  an  irregular  mass  of 
nimbus  cloud,  from  which  a  slight  fall  of  rain  took  place,  the 
drops  appearing  to  be  evaporated  in  the  lower  air  before 
attaining  the  surface  of  the  earth.  We  have  in  this  instance 
a  fair  illustration  of  the  principle  of  the  whirlwind,  and  also  a 
small  instance  of  the  accidents  which  are  to  be  described  in 
the  next  paragraph. 

The  whirlwinds  which  attended  the  great  eruption  of 
Sumbawa,  an  island  in  the  East  Indies,  in  1815,  destroyed 
great  areas  of  forests  and  drew  up  into  the  air  the  bodies  of 
men  and  beasts,  adding  another  source  of  havoc  to  that  dire 
catastrophe.  Where  these  whirls  are  formed  over  the  heated 
surface  of  the  sea  they  are  often  much  more  vigorous  than 
the  similar  movements  on  the  surface  of  the  continental  lands, 
for  the  reason  that  the  air  over  the  sea  often  remains  for  a 
long  time  calmer  than  over  the  land-surfaces.  The  greater 
energy  of  these  whirlwinds  over  the  surface  of  the  sea  may 
also  be  in  part  due  to  the  moister  nature  of  the  air  above  that 
surface,  which  brings  about  an  upward  impulse  in  the  column 
— in  a  manner  to  be  noted  hereafter.  Where  strong  whirl- 
winds occur  over  the  surface  of  the  sea  they  produce  the 
phenomena  called  water-spouts.  The  common  notion  that 
these  marine  whirlwinds  suck  up  water  from  the  sea  to  the 
clouds  is  almost  certainly  an  error.  It  is  true  that  the  water 
leaps   to    the    height   of    a   few   feet    above    the    surface   just 


234 


ASPECTS    OF    THE  EARTH. 


^'-'■'-^J'J  %-. 


beneath  the  central  part  of  the  cokimn,  but  the  cyHnder  of 
cloud  is  due  to  the  rapid  condensation  of  the  moisture  in  the 
air  which  is  drawn  up  through  its  centre- — condensation  pro- 
duced by  the  cooling  which  the  air  receives  as  soon  as  it 
escapes  from  the  thin,  heated  lower  layer.  As  we  shall 
shortly  see,  the  prairie  tornado  has  the  same  general  aspect 
as  the  water-spout,  though  there  is  no  sea  below  it  from 
which  it  can  draw  its  water. 

The  passage  from  the  sand-whirls  of  the  streets  and  other 

desert  places  to  the 
tornadoes  such  as 
ravage  the  central 
part  of  this  country 
appears  at  first  sight 
to  be  gradual  ;  yet,  as 
we  shall  see,  though 
both  depend  upon 
the  up-rush  of  the 
warm  air  throusfh 
the  colder  overlying  mass,  the  conditions  which  produced 
the  warmth,  and  thereby  give  rise  to  the  current,  are  not 
exactly  the  same.  The  smaller  dust-whirls  occur  every- 
where in  the  world  ;  tornadoes  are  limited  to  particular 
regions,  and  those  of  disastrous  violence  occur  only  in  certain 
limited  parts  of  the  earth's  surface.  One  of  their  seats  of 
most  energetic  development  is  in  the  central  and  western 
parts  of  the  Mississippi  valley.  They  are  peculiarly  frequent 
in  the  sections  from  Western  Ohio  to  Colorado,  though  they 
occur  occasionally  in  about  all  the  level  portions  of  the  cen- 
tral trough  of  the  continent,  and  also  on  the  Atlantic  slope. 
They  happen   most  frequently  in  the  months  of  May,  June, 


-4^- 


A   Water-spout. 


THE  IXSTABILITY  OF  THE  ATMOSPHERE. 


235 


and  July,  but  they  occasionally  occur  at  other  seasons  ;  in- 
deed, they  have  been  observed  in  every  month  in  the  year. 
They  are  commonest  in  the  afternoons,  but  have  been  ob- 
served at  other  times  in  the  day. 

The  way  in  which  these  tornado-whirls  are  formed  differs 
in  certain  essential  particulars  from  the  way  in  which  whirl- 
winds are  created,  as  has  been  well  shown  by  Professor 
Ferrel.  The  most  important  points  of  difference  are  as  fol- 
lows :  The  dust-whirls 
are  due  to  the  heating 
of  a  thin  layer  of  air 
next  the  ground.  The 
small  mass  of  this  layer 
prevents  its  upward 
whirling  from  bringing 
about  any  powerful 
movements  of  the  at- 
mosphere. In  the  tor- 
nado the  heat  of  the 
lower  air  has  a  differ- 
ent origin.  When  a 
cyclone  passes  over  the 

surface  of  a  country,  certain  peculiar  movements  of  the 
atmosphere  which  it  produces  bring  large  volumes  of  the 
warm  and  moistened  air  to  the  earth's  surface  and  overlay 
them  by  a  cool  stratum.  It  is  not  necessary  for  us  to  de- 
scribe the  exact  process  by  which  this  condition  is  brought 
about  ;  it  depends  upon  rather  complicated  reactions  which 
take  place  within  the  cyclonic  whirl.  It  is  sufficient  for  our 
purpose  to  note  that  in  this  manner  a  deep  layer  of  warm  air 
is  placed  next  the  surface  of  the  earth,  and  that  it  does  not 


Section  through  a  Tornado. 


236  ASPECTS   OF  THE  EARTH 

owe  its  temperature  in  any  immediate  way  to  the  heat  which 
radiates  from  the  earth's  surface.  This  layer  of  warm,  moist 
air  tends  to  rise  up  for  the  same  reason  that  the  thin  layer  of 
dry  air  which  forms  the  dust-whirl  is  impelled  upward,  but  on 
account  of  its  great  mass  the  intensity  of  the  upward  urgence 
is  far  greater. 

In  the  sand-whirl  the  upward  motion  begins  close  to  the 
earth's  surface  for  the  reason  that  the  stratum  which  is  im- 
pelled upward  is  very  thin,  but  in  the  tornado  the  stratum  of 
heated  air  is  usually  about  a  thousand  feet  thick  ;  therefore 
its  whirling  action  naturally  originates  at  the  upper  surface  of 
the  hot  layer,  for  it  is  at  that  point  the  upward  motion  begins. 
Starting  in  this  upper  region,  the  whirl  extends  progressively 
downward,  just  as  in  the  bath-tub  the  whirl  extends  progres- 
sively upward  from  the  point  at  which  the  motion  originated, 
until  the  whirl  may  touch  the  surface  of  the  earth.  When 
these  whirls  begin  they  only  involve  a  small  part  of  the  air 
about  the  point  of  origin,  and  so  the  acquired  velocity  of  the 
particles  when  they  come  to  the  centre  is  not  great ;  but  grad- 
ually they  suck  air  from  farther  and  farther  away.  As  the 
field  of  supply  becomes  larger,  and  the  particles  move  from  a 
greater  distance,  they  approach  that  centre  with  greater  and 
greater  speed,  and  the  spiral  widens  and  turns  with  accel- 
erated velocity.  The  longer  the  journey  of  the  particle,  the 
swifter  its  whirling  motion  becomes.  We  may  secure  a 
familiar  and  fairly  good  illustration  of  this  motion  by  whirling 
a  weight  on  a  string  and  at  the  same  time  allowing  the  string 
to  coil  around  the  finger,  thus  constantly  shortening  the 
length  of  the  circuit  the  weight  traverses.  We  thus  observe 
that  the  speed  of  the  motion  sensibly  increases  as  the  line 
shortens. 


THE  INSTABILITY  OF   THE  ATMOSPHERE.  237 

When  the  particles  of  air  start  from  a  mile  away  toward 
the  centre  of  the  whirl,  they  may  move  at  the  rate  of  a  gentle 
breeze  ;  when  they  have  come  to  within  a  hundred  feet  of  the 
centre,  the  motion  may  have  the  speed  of  a  hurricane.  The 
more  nearly  the  particle  of  air  approaches,  the  stronger  the 
centrifugal  force  becomes,  and  the  air  pulls  away  from  the 
centre  just  as  does  the  weight  attached  to  the  string  when  it 
is  coiled  nearly  to  the  finger.  The  result  is  a  partial  vacuum 
in  the  centre  of  the  tornado-shaft  which  seeks  to  be  filled.  It 
must  fill  itself  from  either  end  of  the  cavity.  It  cannot  draw 
air  from  above,  for  the  reason  that  there  the  atmosphere  is  so 
much  lighter  that  it  will  not  descend,  but  on  the  surface  of  the 
ground  there  is  air  which,  though  whirling,  is  not  moving  with 
anything  like  the  speed  that  it  has  in  the  higher  part  of  the 
shaft,  for  the  following  reasons  :  In  the  first  place,  the  whirl 
begins  high  up  and  extends  gradually  downward  toward  the 
earth's  surface,  therefore  the  air  next  the  ground,  being  the 
last  to  be  set  in  motion,  has  not  acquired  the  speed  of  that  in 
the  upper  portions  of  the  column  ;  furthermore,  the  air  upon 
the  surface  is  hindered  in  its  movements  by  the  great  friction 
which  the  irregularities  of  the  earth  exert  upon  it — this  fric- 
tion in  a  tornado,  as  in  an  ordinary  gale,  reducing  the  rate  of 
the  motion  in  a  surprising  manner.  The  reader  may  readily 
observe  this  effect  by  noting  the  speed  with  which  the  scud  of 
a  storm  flying  at  perhaps  a  thousand  feet  above  the  surface 
moves.  He  will  often  find  a  motion  of  fifty  miles  an  hour  or 
more  indicated  by  this  scud,  while  on  the  surface  of  the  earth 
the  speed  of  the  gale  does  not  exceed  half  that  amount.  This 
difference  represents  the  effect  of  the  earth's  friction.  The 
result  is  that  this  relatively  quiet  air  next  the  ground  is  sucked 
into  the  tube  with  extreme  rapidity,  and  mounts  with  much  less 


238 


ASPECTS   OF  THE  EARTH. 


whirling  movement  than  we  find  in  what  we  may  term  walls  of 
the  whirl — that  is,  the  rapidly  circling  particles  which  lie  on 
either  side  of  the  partly  vacant  central  portion  of  the  column. 
Curiously  enough,  the  up-rushing  air  in  the  central  shaft  of 
the  tornado  obtains  a  certain  access  of  heat  from  the  upward 
motion  of  the  atmosphere  in  the  shaft.      This  gain  of  force  is 


Effect  on  a  Train    in   the  Centre  of  a  Tornado.      From  a  photograph   taken  at   Sauk  Rapids,  Minn.,  April,  1886. 

brought  about  in  the  following  manner:  The  warm  air,  the 
rush  of  which  constitutes  the  tornado,  contains  a  considerable 
amount  of  water  in  the  form  of  vapor.  This  water  is  held  in 
the  vaporous  form  by  the  action  of  the  heat,  which  pushes  its 
molecules  apart.  As  soon  as  anything  causes  this  vapor  to 
condense  in  the  form  of  visible  water,  the  force  which  pushed 
the  molecules  asunder  again  appears  as  heat,  and,  by  expand- 
ing the  air  in  which  the  condensation  takes  place,  causes  it  to 
retain   its  ascending  force  for  a  greater   time   than  it   would 


THE  INSTABILITY  OF   THE  ATMOSPHERE.  239 

Otherwise  maintain  it,  and  so  intensifies  and  continues  the 
uprushing  movement  of  the  column.  In  the  ordinary  tornado, 
owing  to  its  relatively  small  size,  and  to  the  brief  duration  of 
its  action,  this  force  derived  from  the  condensation-vapor  has 
no  very  great  influence  on  the  violence  of  the  movement ;  but, 
as  we  shall  hereafter  see,  this  peculiar  effect  of  condensing 
vapor  has  a  great  importance  in  the  cyclones,  that  last  species 
of  atmospheric  whirls  which  we  have  yet  to  consider. 

When  the  conditions  of  atmospheric  instability  have  given 
birth  to  a  tornado,  the  fact  is  announced  to  the  observer  by 
a  sudden  gathering  of  dark,  swift-whirling  clouds,  from  which 
depends  a  writhing,  serpent-like  body  formed  of  condensed 
vapor.  This  writhing  column  extends  rapidly  downward  until 
it  touches  the  earth.  When  it  attains  the  surface  it  becomes 
audible  from  the  violent  rending  actions  which  it  creates  upon 
that  surface.  As  soon  as  the  whirl  is  created  it  begins  to 
move  away,  generally  toward  the  northeast, — for  the  evident 
reason  that  the  upper  cold  layer  of  air  against  which  it  origin- 
ates has,  in  the  northern  hemisphere,  a  movement  in  that 
direction. 

In  its  path  over  the  surface,  the  circling  movement  of  the 
writhing  air  and  the  sucking  action  of  the  partial  vacuum  in 
the  central  portion  of  the  shaft  combine  to  bring  about  an 
extreme  devastation.  On  the  outside  of  the  whirl  the  air, 
which  rushes  in  a  circling  path  toward  the  vortex,  overturns 
all  movable  objects,  and  in  the  centre  these  objects,  if  they  are 
not  too  heavy,  are  sucked  up  as  by  a  great  air-pump.  Thus 
the  roofs  of  houses,  bodies  of  men  and  animals,  may  be  lifted 
to  great  elevations,  until  they  are  tossed  by  the  tumultuous 
movements  beyond  the  limits  of  the  ascending  currents  and 
fall  back  upon  the  earth.     Where  the  centre  of  the  whirlwind 


240 


ASPEC7S   OF  THE  EARTH. 


passes  over  a  building,  the  sudden  decrease  in  the  pressure 
of  the  outer  air  often  causes  the  atmosphere  which  is  con- 
tained within  the  walls  suddenly  to  press  against  the  sides  of 
the  structure,  so  that  these  sides  are  quickly  driven  outward 
as  by  a  charge  of  gunpowder. 

It  is  not  unlikely  that  the  diminution  of  pressure  brought 


"^•-., 


Showing  Explosive  Effect  of  Air  contained  in  the  Hollow  Wall  of  a  Building.      From  a  photograph  taken  at 
Rochester,  Minn.,  August,  1883.      [Note  that  the  effect  is  limited  to  a  small  part  of  the  edifice.] 

about  by  the  passage  of  the  interior  of  the  whirl  over  a  build- 
ing may  be  about  as  much  as  is  indicated  by  the  fall  of  four 
inches  in  the  barometer.  This  is  equivalent  to  a  change  in 
the  pressure  amounting  to  about  three  hundred  pounds  to  the 
square  foot.  This  force  operates  to  burst  out  the  walls  of  a 
building.     It  is  not  improbable  that  the  diminution  of  pressure 


THE  INSTABILITY  OF  THE  ATMOSPHERE. 


241 


may  be  much  greater  than  this,  but  even  the  amount  named  is 
sufficient  to  account  for  the  destruction  of  the  frail-walled 
structures  which  these  devastating  movements  encounter  in 
the  western  parts  of  the  United  States. 

Fortunately  the  paths  of  these  tornadoes  are  ordinarily 
very  narrow — the  widest  have  a  diameter  of  less  than  two 
miles;  the  narrowest  of  only  forty  feet.      In  most  cases  a  tor- 


Showing  the  Narrow  Limits  of  the  Destruction  and   the  Completeness  of  the    Ruin  within  the    Limited    Field. 
From  a  photograph  taken  at  Rochester,  Minn.,  August,  1883. 


nado  is  seriously  destructive  over  a  width  not  exceeding  five 
hundred  feet.  The  length  of  the  tornado's  path  across  the 
country  does  not  commonly  exceed  thirty  miles,  and  it  gener- 
ally traverses  the  distance  in  about  an  hour.  When  the  up- 
ward corkscrew  motion  of  the  outer  part  of  the  spiral  and  the 
swifter  up-rush  of  the  air  through  the  central  shaft  have  drained 
away  the  most  of  the  warm  air  which  gave  birth  to  the  motion, 
the  tornado  dies  away.  The  equilibrium  of  the  air-masses  is 
for  a  time  restored,  the  heavier  air  has  fallen  down  upon  the 


242  ASPECTS   OF  THE  EARTH. 

surface,  and  the  warm  air,  spreading  laterally  as  it  attains  the 
level  to  which  it  tends,  comes  into  a  state  of  quiet.  Assuming 
the  width  of  the  destruction  brought  about  by  the  storm  at  six 
hundred  feet,  and  the  length  of  its  journey  at  thirty  miles,  we 
find  that  the  area  of  its  devastation  amounts  to  about  two 
thousand  acres,  or  to  a  square  area  about  two  miles  on  a  side. 
Over  this  area  the  destruction  is  ordinarily  more  complete  than 
that  which  occurs  in  the  most  severe  earthquakes.* 


*  These  tornadoes  are,  even  in  the  present  scattered  condition  of  the  population 
in  the  regions  they  afflict,  a  source  of  great  destruction  to  life  and  property,  and  with 
the  increase  of  population  each  year  they  are  likely  to  produce  even  greater  loss.  The 
question  arises,  What  can  be  done  to  mitigate  these  evils  ?  It  is  evident  that  these 
devastations  depend  upon  such  great  causes  that  we  cannot  hope  in  any  manner  to 
prevent  their  occurrence,  but  it  seems  possible  in  certain  simple  ways  to  limit  the 
destruction  they  bring  about.  By  far  the  greatest  loss  of  life  and  property  is  caused 
by  the  frail  nature  of  the  structures— generally  timber  buildings  of  unsubstantial 
character— in  which  inhabitants  of  the  tornado  district  dwell.  These  buildings, 
though  well  suited  to  resist  the  action  of  earthquakes,  are  utterly  unfitted  to  oppose 
these  convulsions  of  the  air:  A  building  intended  to  meet  the  tornado  shock  should, 
it  seems  to  me,  be  constructed  in  the  following  manner  :  Where  possible,  it  should 
possess  thick  masonry  walls  of  stone  or  brick  united  by  strong  mortar.  Masonry 
seems  to  be  the  preferable  material,  for  the  reason  that  the  storm,  owing  to  its  rapid 
forward  movement,  acts  on  any  one  place  having  the  area  of  a  house  for  only  a 
second  or  two  ;  thus  the  inertia  of  the  mass  will  serve  to  protect  it  from  the  ravage  of 
the  brief  storm.  If  there  are  partition  walls  within  the  house,  these  partitions  should 
be  tied  firmly  to  the  outer  walls  by  suitable  bolts.  There  should  be  large  windows 
in  the  cellars  and  in  the  house  itself,  which  may  be  blown  out  with  ease,  and  so  afford 
ecrress  to  the  expanding  air.  Roofs  should  be  firmly  tied  to  the  outer  and  inner 
walls,  and  the  attic  space  should  be  provided  with  windows  which  would  similarly 
permit  the  egress  of  the  air.  The  building  should  be  of  as  little  height  as  possible. 
There  should  be  no  external  parts  of  the  edifice  which  are  not  well  secured  to  the 
main  mass.  Timber  fences  and  other  frail  structures,  which  are  easily  torn  to  pieces 
by  the  storm,  may  supply  debris  with  which  the  wind,  by  whirling  about,  may  inflict 
damage.  Such  a  house  would  be  likely  to  survive  the  action  of  almost  all  the 
observed  tornadoes.  It  would  be  well,  however,  for  the  occupants  of  even  the  best 
constructed  houses  in  districts  much  afflicted  by  tornadoes  to  have  a  refuge-chamber 
constructed   a  little  below  the  surface  of  the  ground,  immediately  adjacent  to  the 


s  of  a  Tornado. 


From  photographs  taken  near  Jamestown,  Dak.,  June  6,  1887,  by  Mr.  C.  L.  Judd,  while  the  column  was  eigh- 
teen miles  distant  and  rapidly  receding.  The  upper  picture  represents  the  tornado  at  its  fullest  vigor ;  the  lower, 
when  it  had  begun  to  wane.  The  centre  is  shown  by  the  dark  line  of  the  funnel,  behind  which  trails  the  storm  of 
rain  and  hail  which  is  a  usual  accompaniment.  In  passing  over  a  lake  about  two  acres  in  area,  this  tornado 
sucked  up  all  the  water,  leaving  the  ground  "  dry  enough  to  be  ploughed." 


THE  IXST ABILITY  OF  THE  ATMOSPHERE.  243 

We  have  already  noted  the  fact  that  these  tornadoes  are 
due  to  the  presence  of  thick  masses  of  warm  and  moist  air  next 
the  surface  of  the  earth  which  seeks  a  passage  up  through  the1 
superincumbent  atmosphere.  Recent  discoveries  have  made 
it  clear  that  these  destructive  whirlwinds  lie  within  the  field 
of  certain  greater  whirls,  known  as  cyclones,  and  that  to  the 
action  of  these  vast  revolving  storms  we  owe  the  atmospheric 
conditions  which  lead  to  the  tornadoes.  The  peculiar  move- 
ment of  these  cyclones  crowds  great  masses  of  warm  air  toward 
the  southeastern   portion   of  their   border,  which   masses  are 

south-west  side,  and  readily  accessible  from  the  interior  as  well  as  from  the  exterior 
of  the  dwelling,  to  which  they  may  resort  upon  the  approach  of  the  danger.  An 
underground  chamber,  eight  feet  square  and  six  feet  high,  covered  by  three  or  four 
feet  of  earth,  provided  with  one  or  two  entrances  of  no  more  than  sufficient  size, 
without  doors,  would  afford  an  absolutely  safe  refuge  in  the  worst  of  these  catas- 
trophes. 

The  records  of  Western  tornadoes  show  within  the  last  ten  years  a  loss  of  killed 
and  wounded  of  between  one  and  two  thousand  persons.  By  far  the  greater  part  of 
these  accidents  to  life  and  limb  might  have  been  avoided  if  such  provisions  for  refuge 
had  existed.  The  loss  of  life  from  lightning  in  the  same  region  has  not  been  any- 
thing like  as  great,  and  yet  almost  every  house  has  its  provision  of  rods,  which  are 
much  more  costly  than  the  storm-refuges  which  we  have  described,  and  are  gener- 
ally worthless  for  protection. 

In  the  case  of  barns  the  part  devoted  to  sheltering  stock  should  be  placed  partly 
underground,  and  the  portion  above  the  surface  should  be  banked  up  with  earth  as 
high  as  may  be.  The  floor  which  parts  the  level  of  the  stabling  from  the  upper  por- 
tion should  be  strongly  secured  to  the  lower  walls.  In  this  way  the  upper  portion  of 
the  building  may  be  abandoned  to  the  chance  of  accident,  while  the  part  containing 
the  beasts  may  be  secure. 

It  is  quite  conceivable  that  something  may  be  done  by  means  of  telegraphic 
communication  to  convey  intelligence  concerning  the  movements  of  these  tornadoes, 
but  the  warning  given  by  the  roar  of  the  movements  upon  the  surface  is,  except  in 
the  rare  cases  where  the  catastrophe  occurs  in  the  night-time,  sufficient,  when  taken 
with  the  long  forewarning  afforded  by  the  aspect  of  the  sky,  to  put  people  on  their 
guard.  The  time  is  generally  ample  for  men  to  return  from  the  field  and  place 
themselves  and  their  beasts  in  their  respective  shelters. 


244 


ASPECTS   OF   THE  EARTH. 


overrun  by  the  cooler  upper  atmosphere,  thus  bringing  about 
conditions  which  give  birth  to  the  tornado.  It  is  readily  seen 
that  this  discovery  may  make  it  possible  for  the  meteorologist 
to  predict,  at  least  in  a  general  way,  the  districts  which  are 
liable  to  tornadoes,  but  it  is  still  far  beyond  his  science  to  tell 
just  where  the  blow  will  be  struck. 

There  are  many  reasons  why  it  is  desirable  to  know  some- 
thing concerning  the  distribution  of  tornadoes  in  this  country. 


Showing  the  Overturning  Action  of  a  Tornado  on    Buildings.      Fronn  a  photograph  taken  at  St.  Cloud,  Minn., 

April  15,  1886. 

As  yet  a  large  part  of  the  field  subject  to  frequent  cataclysms 
of  this  nature  has  been  settled  for  such  a  short  time  that  it  is 
not  possible  to  secure  satisfactory  data  on  which  to  found  any 
statements  concerning  the  frequency  of  their  occurrence.  The 
incomplete  evidence,  however,  goes  to  show  that  nearly  all  of 
the  region  of  the  eastern  United  States,  say  from  the  one  hun- 
dredth meridian  eastward  to  the  Atlantic,  is  more  or  less  liable 
to  visitations  of  this  nature.  It  is  evident,  however,  that  in 
their  frequency  of  occurrence,  as  well  as  in  their  intensity  and 
the  range  of  their  destructive  paths,  they  vary  greatly  in  dif- 


THE  INSTABILITY  OF   THE  ATMOSPHERE.  245 

ferent  parts  of  this  great  area.  They  are  most  frequent  and 
most  intense  in  the  untimbered  districts  of  the  Mississippi 
valley  north  of  the  Red  River.  It  may  be  that  their  relative 
fury  in  this  field  is  due  to  the  fact  that  the  cyclonic  storms 
which  they  attend  are  most  frequent  and  well-developed  in 
that  section  of  the  field.  It  is  possible  that  the  absence  of 
timber  may  have  something  to  do  with  producing  the  atmos- 
pheric tensions  which  lead  to  these  catastrophes. 

Tornadoes  of  much  violence  occasionally  occur  in  Ken- 
tucky, Tennessee,  and  the  northern  portion  of  the  Gulf  States. 
A  few  such  accidents  have  been  recorded  in  the  Atlantic  States 
north  of  Virginia,  but  their  more  violent  and  frequent  mani- 
festations are  practically  limited  to  that  part  of  the  Mississippi 
valley  which  lies  to  the  north  of  the  Ohio  River  and  the  Red 
River.  In  that  field,  although  the  liability  to  tornadoes  varies 
in  different  parts  of  its  areas,  it  is  safe  to  say  that  these  acci- 
dents are  probably  ten  times  as  frequent  as  in  any  other  equal 
area  on  this  continent. 

In  default  of  historic  records,  it  seems  possible  to  obtain 
some  clew  to  the  hurricane  history  of  any  region  occupied  by 
forests,  by  an  inspection  of  the  conditions  of  its  woodlands. 
The  hurricane  tracks  in  the  woods  of  the  Mississippi  valley 
remain  evident  from  the  fallen  trees  and  the  relative  youth  of 
the  new-grown  timber  for  more  than  a  century  after  the  occur- 
rence of  the  storm.  Even  after  several  centuries  the  influence 
of  the  catastrophe  may  be  indicated  by  the  change  in  the 
character  of  the  timber  along  the  path  of  the  storm.  If  in 
any  woodland  whatsoever  a  belt  of  timber  is  removed,  either 
by  the  axe,  by  fire,  or  by  tempest,  the  trees  which  succeed 
those  which  have  been  destroyed  are  generally  of  different 
species  from  those  which  have  previously  occupied  the  area. 


246 


ASPECTS   OF  THE  EARTH. 


This  phenomenon  of  succession  in  forest  trees  is  familiar  to  all 
those  who  are  well  informed  concerning  the  histories  of  great 
areas  of  woods,  and  the  facts  enable  us  frequently  to  assert 
that  at  some  time  in  the  past,  perhaps  three  or  four  centuries 
ao-o,  a  given  tract  of  country  has  been  swept  by  a  tornado.  At 
several  points  in  Kentucky  and  in  the  neighboring  States  I 
have  been    able    in    this    manner    to  determine  the  track  of 


Showing  Grades   of  Destruction   from   Centre  to  Border  of  Tornado.      From  a  photograph  taken  at  St.  Cloud,, 

Minn.,  April  15,  1886. 

ancient  hurricanes.  My  observations  lead  me  to  the  conclu- 
sion that  devastating  accidents  of  this  description  are  on  the 
whole    rare   throughout   the   timbered   districts   of    the    Ohio 

valley. 

In  cyclones  we  find  the  largest  manifestation  of  that  energy 
by  which  the  superheated  lower  air  whirls  upward  from  the 
earth  through  openings  which  it  has  rent  in  the  higher  cooler 
layers.      In  its  fundamental  cause  the   cyclone  is   essentially 


THE  IXSTABILITV  OF   THE  ATMOSPHERE.  24; 

like  both  the  lesser  classes  of  whirls,  the  dust-whirls  and  torna- 
does, but  the  field  of  its  work  is  vastly  greater,  though  the 
energy  which  it  exercises  at  any  one  point  is  less.  The  con- 
ditions which  lead  to  the  formation  of  a  cyclone  are  probably 
as  follows  :  In  those  heated  portions  of  land  and  sea  where  the 
circumstances  permit  the  air  to  remain  for  a  long  time  undis- 
turbed, it  becomes  very  warm  and  charged  with  moisture  ;  the 
hotter  it  becomes  the  more  moisture  it  contains,  and  the  less  it 
permits  the  heat  radiating  from  the  surface  to  pass  through  its 
texture  ;  at  the  same  time  the  upper  air,  deprived  of  its  usual 
share  of  radiant  heat,  becomes  abnormally  cold  ;  finally,  as  in 
the  dust-whirls  and  tornadoes,  the  lower  air  breaks  through  the 
upper  and  rushes  toward  the  sky.  Although  at  its  beginning 
a  cyclonic  storm  is  probably  of  no  greater  size  and  of  much 
less  ascending  force  than  a  tornado,  there  are  several  reasons 
which  make  its  history  different  from  that  of  the  smaller 
whirls.  In  the  first  place,  the  field  of  heated  air  which  causes 
the  cyclone  is  far  more  extensive  than  that  which  produces  the 
tornado,  though  at  the  same  time  the  difference  of  tempera- 
ture between  the  upper  and  lower  air  may  be  less.  The 
greater  bulk  of  the  lower  stratum  of  hot  and  moist  air  permits 
the  cyclone  to  grow  larger,  but  the  less  ascensional  force  of 
the  lower  air  makes  it  rather  less  violent  in  its  movements. 

.  As  soon  as  the  ascending  current  brings  a  portion  of  the 
heated  air  from  the  surface  into  the  higher  level,  it  expands, 
and  the  force,  originally  in  the  form  of  heat,  which  kept  it  in 
the  state  of  vapor,  serves  to  increase  the  ascending  column 
just  as  much  as  would  the  direct  application  of  heat  sufficient 
to  vaporize  the  water.  Thus  we  have  two  sources  of  force  to 
impel  the  air  in  the  cyclone  upward.  Both  these  forces,  as 
we  have   already  seen,  appear  in   the  tornado,  but  there  the 


248 


ASPECTS   OF  THE  EARTH. 


original  heat  of  the  lower  air  is  the  principal  cause  of  the 
motion.  The  heat  arising  from  the  condensation  of  vapor  is 
of  considerable  moment  in  cyclones,  especially  those  which 
occur  over  tropical  seas.  Torrential  rains  fall  beneath  the 
wide  central  shaft  of  the 
storm,  and  every  particle 
of  the  falling  water  repre- 


Showing  Sharp   Passage  from   the  Centre  to  the  Periphery  of  a  Tornado.      From  a  photograph  taken  at 

St.  Charles,    Mo. 

sents  the  conversion  of  energy  which  held  the  fluid  in  the 
shape  of  vapor,  into  force  which  is  added  to  the  essential 
vigor  of  the  up-rush  of  air.  To  this  cause  we  may  perhaps 
attribute,  in  part  at  least,  the  long  life  of  these  cyclones, 
and  the  great  size  to  which  their  whirls  attain.  Unlike 
the    tornadoes,  they  often    continue    in    existence    for    many 


THE  INSTABILITY  OF   THE  ATMOSPHERE.  249 

days,  have  a  width   of  several  hundred  miles,  and  sometimes 
pass  over  a  course  several  thousand  miles  in  length.  | 

As    in   the    case   of   the   dust-whirl    and   the    tornado,    the 
ascendino-  column   of  air,  after  attaining   the  height  where  it 
no  longer  tends  to  rise  upward,  spreads  out  over  the  surface 
of  the  sheet  through  which   it  has  broken  its  way.      When  it 
has  drained  out  all  the  air  warm  enough  to   rush  upward  the 
disturbance  ceases.     All  these  larger  whirling  movements  of 
the  air,  whether  they  occur  on  land  or  sea,  move  forward  in 
directions  proper  to  the  region  in  which  they  occur,  at  a  more 
or  less  rapid  rate,— in   the  cyclones  these   translatory  move- 
ments of  the  storm  being  sometimes  at  the  rate  of  fifty  miles 
an    hour.       The   principal    cause   determining  the   speed    and 
direction   of  the   movement  is  doubtless  the  course  of  flow  of 
the  great  upper  currents  of  the  atmosphere,  which,  however 
perfect  the  calm  of  the  surface,  are  always  in  motion  in  deter- 
mined directions.     This  element  of  regularity  in   the   move- 
ment of  cyclones  enables  us  to  predict,  in  some  regions  with 
areat  certainty,  the  direction   in  which  these  whirls  will  move. 
Observations  have  also  determined  the  regions  where  storms 
of  this   nature  occur,  and  the  seasons  of  the  year  when  they 
may  be  expected.      Science  has  gone   still  further,  and  shown 
the    mariner   how   he    may   in    most    cases    avoid    the   central 
portions  of  the  storm-area,  and  so  escape  the   dangers  arising 
from   the   strongest  winds.*     The    rotation   of    the   earth    so 

*  The  Ibllowing-  account  of  the  rules  for  avoiding;  the  storms  is  extracted  from 
Professor  W.  M.  Davis's  Whirlwinds,  Cyclones,  and  Tornadoes:  "The  storm's 
earliest  effect  on  the  atmosphere  is  shown  by  the  barometer.  It  is  ordinarily  stated 
that  the  first  effect  is  seen  in  a  diminution  of  pressure  ;  but  it  is  very  probable,  both 
from  theory  and  from  careful  observation,  that  a  slight  abnormal  increase  of  pressure 
precedes  this  diminution.  The  tropical  seas,  where  cyclones  are  most  violent,  have, 
as  a  rule,  very  small  and  very  rare  irregular  changes  in  atmospheric  pressure  ;  and 


250  ASPECTS    OF  THE  EARTH. 

affects  the  movement  of  these  great  spiral  ascending  currents 
that  in  the  southern  hemisphere  they  always  spin  in  the  direc- 
tion in  which  the  hands  of  a  watch  turn  when  it  is  held  hori- 
zontally, with  its  face  toward  the  eye,  while  in  the  northern 
hemisphere  they  move  in  the  reverse  direction.  On  this  gen- 
eral basis,  rules  have  been  laid  down  for  the  direction  of 
mariners  when  they  find  themselves  in  contact  with  these 
storms. 

Recent  studies  on  the  phenomena  of  oceanic  cyclones  have 
served  still  further  to  aid  the  navigator  by  showing  him  some- 
thing of  the  laws  which  control  not  only  the  movement  within 
the  great  space  of  the  circular  storms,  but  also  the  path  of  the 
meteor  over  the  surface  of  the  sea.  It  Is  now  believed  that 
the  first  atmospheric  condition  which  indicates  the  approach 

careful  watching  will  pretty  surely  show  a  rising  barometer,  as  the  annulus  of  high 
pressure  that  surrounds  the  storm  moves  over  the  observer.  The  weather  may  still 
be  clear,  and  the  wind  moderate  and  from  its  normal  quarter  ;  but  this  change  in  the 
glass  demands  renewed  watchfulness.  Let  us  suppose  that  such  an  observation  be 
made  on  board  a  vessel  lying  east  of  the  Lesser  Antilles.  The  chart  shows  the  cap- 
tain that  he  is  in  the  stormy  belt.  He  may  be  directly  in  the  path  of  the  advancing 
storm,  where  he  will  feel  its  full  violence  ;  and  he  must  make  the  best  of  his  way 
out  of  it.  Following  the  rising  pressure,  three  other  signs  of  increasing  danger  may 
be  observed :  First,  faint  streamers  of  high  cirrus-clouds  may  be  seen  slowly 
advancing  from  the  southeast  to  the  northwest,  or  from  the  east  to  the  west,  in  the 
high  overflow  from  the  storm's  centre  ;  this  un propitious  change  may  accompany  the 
rising  of  the  barometer,  or  may  be  first  seen  when  the  barometer  is  highest.  Second, 
the  barometer  begins  to  fall,  slowly  at  first,  but  more  and  more  quickly  when  it 
reaches  and  passes  twenty-nine  inches  ;  the  vessel  is  then  within  the  limits  of  the 
storm.  Third,  the  wind  has  shifted  so  as  to  blow  from  a  distinctly  northern  quarter, 
and  its  strength  goes  on  increasing  ;  this  is  the  indraught,  blowing  spirally  toward 
the  centre.  There  is  then  no  longer  any  question  that  a  storm  is  approaching  ;  and 
as  soon  as  a  heavy  bank  of  clouds  makes  itself  seen,  moving  southward  across  the 
eastern  horizon,  then  the  central  part  of  the  storm  is  in  sight.  These  clouds  are  the 
condensed  vapor  in  the  rising  central  spirals,  and  rain  is  falling  from  them.  In 
deciding  on  a  course  to  be  pursued,  the  first  point  to  be  determined  is,  where  is  the 


Showing  Grades  of  Destruction  from  Centre  to   Periphery  of  Tornado.      From  a  photograph  taken  at  St.  Cloud,  Minn., 
April  15,  1886.      [Note  the  relative  immunity  of  the  trees.] 


THE  IXSTABILITV  OF  THE  ATMOSPHERE.  251 

of  a  hurricane  is  found  in  the  cool,  dry,  brisk  winds  and  a 
very  transparent  atmosphere,  accompanied  by  a  rather  unusu- 
ally high  barometer.  When  with  these  conditions  there  is  a 
long  swell  upon  the  sea  proceeding  from  a  point  on  the  hori- 
zon where  light  streamers  of  cloud  rise  up,  the  mariner  may, 
if  the  season  be  between  the  ist  of  June  and  the  ist  of  No- 
vember, and  he  is  placed  in  the  Atlantic  north  of  the  equator, 
be  pretty  sure  that  he  is  upon  the  periphery  of  such  a  great 
storm,  though  still  perhaps  at  a  distance  where  proper  use  of 
his  wits  may  enable  him  to  escape  from  it.  He  may  safely 
assume  that  the  storm  has  a  width  of  from  two  hundred  to 
five  hundred  miles,  though  the  dangerous  part  of  its  area  may 
not  exceed  the  smaller  of  these  figures.  He  may  also  assume 
that  the  rate  of  movement  of  the  storm-centre — that  is,  its 
progress  over  the  sea — is  from  twelve  to  twenty  knots  an  hour, 

storm's  centre  ?  That  being  known,  its  proi)al)le  path  can  be  laid  down  with  con- 
siderable certainty  in  this  part  of  the  ocean  ;  and  then,  perhaps,  the  greatest  danger 
may  be  avoided.  But  here  a  very  practical  difficulty  arises.  To  find  the  direction 
of  the  storm-centre,  we  must  know  the  incurving  angle  of  the  wind's  spiral — the 
angle  of  inward  inclination  that  it  makes  with  a  circle  whose  centre  is  at  the  storm's 
centre.  The  earlier  students  of  the  question — Dove,  Redfield,  Reid,  and  Piddington 
— considered  the  course  of  winds  to  be  concentric  circles,  or  inward  spirals  of  very 
gradual  pitch  ;  so  that  they  said  the  inclination  of  the  wind  is  practically  zero,  and 
a  line  at  right  angles  to  its  course  must  be  a  radius  leading  to  the  centre.  Later 
studies  showed  this  to  be  incorrect.  The  inclination  of  the  wind  inward  from  the 
circle's  tangent  was  found  to  vary  from  twenty  degrees  to  forty  degrees  or  fifty  degrees, 
but  it  was  thought  that  this  inclination  was  symmetrical  on  all  sides  ;  so  that,  with 
an  average  inclination  of  thirty  degrees,  the  storm's  centre  must  always  bear  sixty 
degrees  to  the  left  of  the  wind's  course.  Finally,  the  most  recent  results  seem  to 
show  that  the  wind's  course  is  neither  circular  nor  symmetrically  spiral ;  that  the 
wind's  inclination  is  very  distinctly  different  in  different  latitudes,  on  different  sides 
of  the  storm,  in  the  different  conditions  on  sea  and  land,  at  different  distances  from 
the  centre,  and  at  different  altitudes.  In  so  complicated  a  case,  much  judgment 
will  be  required  to  find  where  the  storm-centre  lies." 


252 


ASPECTS    OF  THE  EARTH. 


at  least  in  low  latitudes,  though  it  may  attain  a  higher  rate 
of  translation  as  it  moves  farther  away  from  the  equator. 
The  most  important  point  is  to  determine  the  general  path 
of  the  storm-centre.  In  low  latitudes  in  the  northern  hemi- 
sphere, this  centre  apparently  moves  in  all  cases  first  west- 


Overturned  Train  ;   showing  Effects  at  Some  Distance  frooi  the  Centre  of  a  Tornado.      From  a  photograph. 

w^ard  and  then  northwestward,  and  slowing  in  its  rate  of 
motion  as  it  so  turns.  Finally,  after  describing  this  curious 
curve,  it  turns  to  the  northeastward,  attaining  then  the  velocity 
of  translation  of  twenty  or  thirty  miles  an  hour,  and  at  the 
same  'time  increasing  the  width  of  the  spiral  but  diminish- 
ing the  energy  of  the  wind   movement.      Having  established 


THE  INSTABILITY  OF  THE  ATMOSPHERE.  253 

its  northeast  course,  it  may  continue  to  move  to  the  east- 
ward until  it  attains  the  coast  of  Europe.  Recent  observa- 
tions have  also  shown  that  the  shape  of  the  storm  is  not  at 
once  what  was  first  supposed,  circular,  but  is  elliptical,  the 
greatest  diameter  of  the  ellipse  sometimes  amounting  in  all  to 
twice  the  least  diameter.  Although  these  newly  ascertained 
points  serve  somewhat  to  complicate  the  duties  of  the  ship- 
master and  to  call  for  the  exercise  of  greater  discretion  in 
meeting  these  dangers,  they  are  manifestly  a  contribution  to 
our  knowledge  which  may  diminish  the  perils  of  the  sea. 

Great  as  is  the  damage  done  by  cyclones  on  the  sea,  they 
are  to  our  modern  well-constructed  steamships  no  longer  so 
fraugrht  with  ills  as  in  the  old  times  when  vessels  were  alto- 
gether  propelled  by  the  air.  Our  steamers  are  rarely  wrecked 
by  them,  for  the  reason  that  their  motive  power  is  independ- 
ent of  the  winds.  But  when  these  great  whirls  approach  the 
shores,  especially  where  these  shores  are  low-lying  and  popu- 
lous, the  destruction  which  they  bring  about  is  sometimes 
frightful.  On  the  delta  shores  of  the  Bay  of  Bengal,  where 
these  cyclones  not  infrequently  occur,  the  destruction  of  human 
life  is  very  great.  Since  the  year  1700,  over  half  a  million 
lives  have  been  lost  in  these  catastrophes.  The  principal 
part  of  the  damage  is  brought  about  in  the  following  way  : 
When  the  storm-centre  is  over  the  land,  the  winds  blowing 
toward  that  centre  from  the  sea  heap  up  the  water  against  the 
shore.  The  rise  of  the  ocean-surface  along  the  shore-line  is 
also  favored  by  the  low  barometer  which  prevails  there,  and 
the  relatively  great  atmospheric  pressure  on  the  periphery  of 
the  storm.  These  two  causes  tilt  up  the  water  next  the  shore 
and  force  the  sea  over  the  dikes,  adding  the  destruction  of 
floods  to  that  brought  about  by  the  winds.      Fortunately  the 


2  54  ASPECTS   OF  THE  EARTH. 

conditions  where  these  unhappy  accidents  of  flood  are  to  be 
feared  are  rare. 

On  the  continent  of  North  America  there  is  only  one 
region  where  the  floods  produced  by  hurricanes  are  likely  ever 
to  prove  a  very  serious  danger  to  the  people.  The  coasts  of 
Florida,  both  its  eastern  and  western  shores,  particularly  the 
land  from  Key  West  northward  to  Indian  River,  are  subject 
to  crreat  incursions  of  the  sea  durinor  the  time  when  hurricanes 
are  moving  across  the  peninsula  toward  the  North  Atlantic. 
The  violence  of  the  wind  in  this  section,  though  its  movement 
is  at  great  speed,  does  not  appear  likely  to  endanger  buildings 
of  ordinary  solidity ;  but  the  changes  in  the  pressure  of  the 
barometer  occurring  during  the  passage  of  the  storm,  as  well 
as  the  drift  of  the  sea  produced  by  the  wind  movement,  serve 
to  raise  the  ocean  waters  many  feet  above  their  usual  level. 
I  was  informed  that  along  the  key-bordered  coast  of  Florida 
between  Key  West  and  Key  Biscayne,  the  sea  has  been 
known  to  rise  during  the  passage  of  one  of  these  storms  to 
the  heiofht  of  ten  or  fifteen  feet  above  its  usual  level.  At 
present  this  coast  region  is  essentially  uninhabited,  and  the 
people  are  generally  in  position  where  they  can  secure  refuge 
on  elevated  portions  of  the  coral  reef  above  the  plane  of  the 
inundation.  If,  however,  this  region  should  ever  become 
thickly  peopled,  the  danger  of  catastrophic  invasions  of  the 
sea  in  hurricane  times  would  be  about  as  great  as  along  the 
shores  of  the  Bay  of  Bengal. 

The  principal  atmospheric  disturbances  of  the  United 
States  usually  have  a  more  or  less  cyclonic  character,  but 
they  are  rarely  such  regular  whirls  as  those  which  form  on 
the  ocean.  The  numerous  storms  which  move  eastward  from 
the  plains  at  the  foot  of  the  Rocky  Mountains  generally  have 


THE  INSTABILITY  OF  THE  ATMOSPHERE.  255 

a  distinct  whirling  motion,  derived,  perhaps,  from  an  ascend- 
ing movement.  Still  in  many  cases  circumstances  of  their 
origin  make  it  plain  that  they  cannot  be  caused,  as  in  the 
other  type  of  marine  cyclones,  by  the  presence  of  relatively 
hot  and  moist  air  upon  the  surface.  The  causes  which  pro- 
duce them  have  not  been  well  determined.  It  seems  likely 
that  they  have  been  originated  in  the  Pacific  Ocean,  or  are 
shaped  by  conditions  derived  from  that  little-known  meteoro- 
loo^ical  field.  Althouo^h  our  Weather  Bureau  has  oriven  them 
much  study,  these  great  land  whirls  afford  still  a  wide  field 
for  research. 

As  we  go  from  the  equator  toward  the  north  pole,  the 
influence  of  the  wide  seas  becomes  less  considerable,  and 
the  variety  of  conditions  afforded  by  the  crowded  lands 
greater.  The  result  is  that  the  region  about  the  north  pole 
has  storms  which  are  more  irregular  than  those  which  we 
find  in  lower  latitudes. 

The  foregoing  account  of  the  perturbations  of  our  atmos- 
phere is  altogether  insufficient  to  give  the  reader  more  than  a 
general  account  of  their  primary  conditions.  We  perceive 
that  in  the  main  they  are  due  to  the  action  of  the  atmosphere 
in  resisting  the  escape  of  radiant  heat,  whereby  its  lower  parts 
become  too  much  heated  to  remain  on  the  surface.  Although 
these  disturbances  are  often  destructive  to  life,  they  arise 
from  the  operation  of  a  mechanism  upon  which  the  existence 
of  all  life  depends.  If  the  air  did  not  thus  retain  the  heat 
which  comes  from  the  sun,  the  earth's  atmosphere  would  rest 
upon  land  and  sea  locked  in  eternal  frost.  As  the  earth- 
quakes are  movements  of  adjustment  which  attend  the 
changes    of    the    crust, — changes    which    preserve    our    lands 


256 


ASPECTS   OF  THE  EARTH. 


above  the  level  of  the  ocean, — so  these  disturbances  of  the 
air  are  apparently  inevitable  actions  arising  from  conditions 
M^hich  are  essentially  beneficent/' 


*The  reader  who  desires  a  sufficient  and  easily  comprehensible  account  of  these 
whirling  movements  cannot  do  better  than  read  the  excellent  book  by  Professor 
Davis  before  referred  to.  If  he  can  use  the  higher  mathematics,  he  will  find  Pro- 
fessor W.  Ferrel's  Recent  Advances  in  Meteorology,  in  the  Annual  Report  of  the 
Chief  Sio-nal  Officer  for  1885,  Appentlix  71,  a  complete  discussion  of  the  subject. 


y-f- 


Effect  on  a  Train  close  to  the  Centre  of  a  Tornado.     From  a  photograph 
taken  at  Grinnell,  la.,  June  i8,  1882. 


FORESTS  OF  NORTH  AMERICA. 


Arboreal  Ancestry  of  Man  ;  Need  of  Destroying  the  Forests  ;  Evil  Effects  of  Deforesting ; 
Relation  of  Forests  to  Soil  ;  Protective  Effects. — Origin  of  Forest  Trees  ;  Geologic  Suc- 
cession of  Plants  ;  Forests  of  Coal  Measures  ;  Evolution  of  the  Form  ;  Comparison  of 
Broad  and  Narrow  Leaved  Trees. — Study  of  a  Forest  District  ;  Purity  of  Forest  .Streams  ; 
Compared  with  those  of  Tilled  Districts. — Soil  of  Forest. — Forest  Sponge  Effect  on  Rain- 
fall ;  Instances  from  Appalachian  Forests.— Variety  of  Trees  in  Forests  of  North  America  ; 
Comparison  with  Europe  ;  Advantages  arising  from  this  Variety. — Bald  Cypress  ;  its 
j^nees. — Sour  Gum  ;  its  Root  Loops. — Willows. — Effect  of  Position  on  Trees. — Recovery 
of  Land  by  Forests  ;  in  Southern  States  ;  in  New  England.— Comparative  Vigor  of  Coni- 
fers and  other  Trees. — Effects  of  Glacial  Period  on  Forests  ;  Processes  of  Selection. — 
Origin  of  Prairies  ;  Effect  of  Fires  ;  Reforesting  of  Prairies — Underground  Work  of 
Forests  ;  Effects  of  Carbonic  Acid  Gas. — Economic  Value  of  Forests  ;  Effects  of  Deforest- 
ino-  on  American  Rivers  ;  Remedial  Measures. — Present  Condition  of  American  Forests. 

The  history  of  mankind  has  been  at  all  times  much  af- 
fected by  the  forest  covering  of  the  earth.  Modern  science 
teaches  that  man  himself,  at  least  so  far  as  his  organic  body 
is  concerned,  is  derived  from  a  long  line  of  creatures  who 
dwelt  in  trees.  His  slender,  agile  body  and  his  delicately 
constructed,  flexible  hand  owe  their  essential  features  to  the 
arboreal  habit  of  his  ancestors.  It  is  also  possible  that  the 
forest  habit  has  left  its  impress  on  man's  mind  as  well  as  his 
body  ;  for,  as  appears  from  a  consideration  of  the  existing 
tree-dwelling  species  of  mammals,  they  are  generally  more 
social,  sympathetic,  and  quicker-witted  animals  than  most  of 
those  who  dwell  upon  the  surface  of  the  earth.  When  the 
brute  passed,  by  some  as  yet  unexplained  gradations,  into 
the    primitive    man,    the    boughs    were    abandoned,    and    the 


258  ASPECTS    OF  THE  EARTH. 

creature  became,  in  a  measure,  changed  to  suit  the  needs 
of  the  firmer  earth.  For  a  while,  however,  the  forest  re- 
mained his  fittest  dwelling-place.  The  tropical  woods,  where 
man  developed,  afforded  varied  food,  and  the  trees  a  ready 
shelter  from  wild  beasts  of  prey. 

It  is  a  most  interestinor  fact  that  the  earliest  of  known 
mammalia,  the  most  primitive  form  of  those  creatures  which 
give  suck  to  their  progeny,  appears  to  have  been  a  tree- 
climbing  form,  and  for  a  very  long  period  in  the  earth's 
history  these  pouched  mammals,  related  to  the  kangaroo  in 
all  which  concerns  the  nurture  of  their  offspring,  carried  the 
thread  of  our  own  life  through  the  manifold  difficulties  and 
dangers  which  beset  it.  Thus  the  highest  group  of  animals 
appears  to  have  found  in  the  forests,  during  the  reptilian 
ages,  when  the  surface  of  the  earth  was  possessed  by  a  vast 
array  of  great  predaceous  creatures  of  lower  estate,  a  safe 
place  in  which  to  shelter.  The  realm  of  the  tree-tops  affords 
even  to  this  day  harborage  to  a  very  large  part  of  our  four- 
footed  kindred.  The  leafy  coverts,  the  cavities  in  the  gnarled 
branches,  the  slender  bridges  from  tree  to  tree,  the  spaces 
which  may  be  cleared  at  a  bound,  all  give  a  vantage-ground 
to  creatures  which  live  by  their  wits,  and  have  to  fly  from 
stronger  and  clumsier  enemies.  At  the  same  time  the  array 
of  nuts  and  fruits  or  nutritious  bark  which  may  be  won  in  the 
forest,  and  the  range  of  insects  which  harbor  there,  afford 
plentiful  food. 

It  is  only  when  the  "  progressive  desires,"  which  made 
him.  in  essence,  man,  led  him  a  stage  above  the  low^est  level 
of  humanity,  that  his  ancestral  woods  began  to  prove  a  hin- 
derance  to  him  ;  it  is  only  with  the  beginning  of  agriculture 
that  the  forests  came  to  be  the  obstinate  foe  of  his  advance, 


FORESTS   OF  NORTH  AMERICA.  259 

which  was  so  long  to  lie  across  his  path.  For  thousands  of 
years  thereafter  he  was  compelled  to  be  a  toilful  forest- 
destroyer  ;  from  the  encumbering  woods,  with  scanty  tools- 
stone  axes  and  fire — -he  had  to  win  his  fields,  the  material  for 
his  dwellings,  and  the  fuel  for  his  hearth. 

With  the  relatively  modern  development  of  civilization 
we  are  coming  to  the  third  state  of  the  relation  of  man  to 
forests  ;  a  stage  when  he  finds  that  this  tree-covering  of  the 
lands  is  necessary  for  the  maintenance  of  those  conditions  of 
climate  and  timber-supply  on  which  the  utility  of  the  earth 
to  him  in  good  part  depends.  The  frontiersman,  that  essence 
of  the  practical  man,  is  still  a  slayer  of  woods,  and  believes 
that  he  serves  the  god  of  progress  by  the  sacrifice  of  the 
forest.  But,  as  knowledge  advances,  the  thoughtful  classes 
become  more  and  more  concerned  as  to  the  conditions  of 
this  earth  during  the  centuries  to  come,  when  this  swift- 
advancing  ruin  of  our  woods  shall  have  been  completed. 
Most  persons  will  heartily  agree  that  it  is  our  bounden  dut\- 
to  transmit  the  inheritance  which  we  enjoy  in  the  earth  un- 
impaired to  the  generations  yet  to  be.  It  is,  unhappily, 
impossible  for  us  so  to  manage  the  store  of  utilities  which 
the  earth  affords  that  there  shall  be  no  diminution  of  the 
supply  for  the  ages  to  come.  It  is  probable  that  the  supply 
of  coal  will  in  good  part  have  disappeared  by  the  year  3000  ; 
and  in  the  fourth  millennial  period  of  our  era.  a  time  less 
remote  in  the  future  than  the  birth  of  Christ  in  the  past, 
the  metals  now  in  use  will  have  to  be  won  with  great  diffi- 
culty— if  obtained  at  all.  Still  we  may  trust  the  advance  of 
knowledge  and  skill  to  compensate  for  these  losses  ;  solar 
energy  may  be  trusted  to  afford  heat  and  aluminum  to  take 
the  place  of  iron  ;  and  the  world  may  be  the  better  for  the 


2  6o  ASPECTS    OF  THE  EARTH. 

change  which  forced  a  rustless  metal  and  a  dustless  fuel   into 
use — at  any  rate,  we  see  that  the  supply  of  mineral  resources 
of  the  earth  necessary  for  our  successors  may  be  prolonged 
for  a  time  in  the  future  which  is  long  beyond  our  power  to 
conceive.      It  is  otherwise  with  the  soil-covering  of  the  earth's 
surface.     So  far  as  we  can  see,  that  Is  the  least  enduring  and 
the  least  replaceable  of  any  of  those  features  on  which  the  life 
of  the  earth  depends.      It  is  the  harvest  of  the  past  ;  and  once 
lost,  it  cannot  be  supplied  save  by  the  slow  process  of  the  ages. 
If  we  take  a  handful  of  any  soil  which  is  fit  for  the  use  of 
plants  and  examine  it  with  the  eye  and  yet  more  closely  with 
a   magnifying  glass,  we  see  that   it  consists   of  fragments  of 
stony  matter  in  various  stages  of  decay.     The   nature  of  soil 
and  its  history  is  treated  in  some  detail  in   the   last  chapter 
of  this  volume.      For   our  present  purpose  we  have  only  to 
note  that  it  is  to  the  progressive  decay  of  these  bits  of  rock 
that  we  owe  the  fitness  of  the  soil  for  the   needs  of   plants. 
The  fragments  of  stone  riven  by  various  accidents  from  their 
original    bedding    places    in    the   firm-set    part    of    the  earth, 
journey  slowly  but  continuously  down  the  slopes  of  the  land 
towards   the  sea.      If  the  inclination  of   the  rocks   on   which 
the   material   rests   is  steep,  the  particles  move  forward  with 
speed  towards  the  rivers.      If,  as  usual,  the  slope  on  which  the 
soils  lie  is  gentle,  the  journey  downwards  is  slow  and  inter- 
rupted.     All  soil  may  be  regarded  as  rock  matter  on  its  way 
to  the  sea.     While  on   the  journey  the  processes  of   decay, 
principally  brought  about  by  combination  of  oxygen  and  other 
gaseous  substances  with  rocky  matter,  bring  a  portion  of  the 
soil  substantially  each  year  into  a  state  where  it  may  be  dis- 
solved  in   water  and  so   feed  the  very  numerous  roots  which 
are  everywhere  watching  for  food. 


FORESTS    OF  NORTH  AMERICA.  26 1 

In  the  natural  condition  of  the  earth's  surface,  when  it  is 
covered  with  a  thick  mat  of  vegetation  such  as  we  find  in  the 
forests  or  prairies,  the  soil  moves  downward  so  slowly  that 
before  its  materials  come  to  the  banks  of  the  streams  and  are 
washed  away  as  silt  to  the  sea,  nearly  all  of  the  plant  food 
is  taken  from  the  waste  and  fed  to  vegetation.  When,  how- 
ever, the  natural  coating  of  vegetation  is  stripped  away  and 
the  soil  subjected  to  the  strange  destructive  action  of  the 
plough,  the  rate  of  transit  to  the  river  beds  is  vastly  in- 
creased. It  is  probably  made  more  than  one-hundred-fold 
as  rapid  as  it  is  in  the  state  of  nature.  If  the  slope  be  steep 
a  single  rain  of  torrential  character  may  carry  a  given  amount 
of  soil  further  on  its  way  to  the  sea  than  it  would  have  jour- 
neyed in  a  thousand  years  of  ordinary  conditions. 

The  most  serious  misfortune  connected  with  the  reckless 
destruction  of  our  forests  arises  from  the  loss  of  the  soil 
from  large  areas  of  land,  by  which  regions  naturally  fertile 
have  been  converted  into  deserts  of  irremediable  sterility. 
Already  a  large  part  of  many  fertile  regions  has  been  steril- 
ized in  this  fashion  ;  and  each  year  a  larger  portion  of  this 
infinitely  precious  heritage  of  life  slips  into  the  rivers,  and 
finds  its  way  to  the  sea,  because  we  have  deprived  it  of  the 
protecting  coating  of  vegetation.  Therefore  it  is  not  alone 
on  account  of  the  surpassing  intellectual  interest  that  forests 
present  to  us,  but  also  from  the  gravest  reasons  of  economy, 
that  they  deserve  to  be  attentively  studied.  In  the  following 
pages  we  shall  endeavor  to  set  forth,  though  in  mere  outlines, 
the  general  facts  which  are  known  concerning  forests^ — the 
scientific  as  well  as  the  economic  points  together,  for  they 
are  so  united  that  they  cannot  well  be  separately  treated. 

To  find  the  oricrin  of  forests  we  must  gro  back  to  the  first 


262  ASPECTS   OF  THE  EARTH. 

stages  of  vegetable  life.  The  series  of  plants,  as  well  as  that 
of  animals,  began  in  the  water,  and  came  thence  to  the  sur- 
face of  the  lands.  All  very  lowly  forms  of  organisms  demand 
a  permanent  envelope  of  water,  for  the  reason  that  they  are 
not  provided  with  any  skin  which  will  prevent  the  drying 
out  of  the  fluids  of  which  their  bodies  are  in  large  part  com- 
posed. It  is  only  after  they  have  attained  a  certain  speciali- 
zation of  development  that  they  can  withstand  the  strenuous 
conditions  to  which  they  are  subjected  in  the  atmosphere. 
The  beginnings  of  plant  life  in  the  land  were  laid  in  water 
plants  of  simple  structure  ;  thence  they  came  to  fitness  for 
the  land  conditions  on  one  or  more  lines  of  development. 

The  first  land  plants  of  which  we  have  any  evidence  from 
fossilized  remains  are  forms  allied  to  our  ferns,  which  appear 
in  the  upper  Silurian  age  ;  but  it  is  improbable  that  they 
were  the  earliest  forms  which  dwelt  in  the  air.  It  is  likely 
that  the  lowlier  groups  of  mosses  and  lichens  preceded  them 
in  time,  and  that  we  have  failed  to  find  their  remains.  For 
some  ages  we  have  very  imperfect  records  of  the  ancient 
forests;  but  we  know  that  during  the  Devonian  period  some 
of  the  ferns  had  taken  on  a  tree-like  aspect,  and  probably 
formed  a  low,  bushy  growth  in  the  swamps  of  that  time.  As 
these  ferns  came  to  be  crowded  together  there  began  a  great 
strueele  for  existence,  which  has  continued  to  this  day,  and 
which  has  given  our  forests  their  most  conspicuous  aspect. 
Each  individual  plant  needed  to  attain  a  share  of  sunlight, 
and  so  in  a  way  sought  to  overtop  its  neighbors.  Those 
which  developed  taller  trunks  than  their  competitors  for  light 
prevailed  in  the  struggle  for  existence,  and  transmitted  their 
peculiarities  to  their  descendants;  those  less  endowed  in  this 
respect  generally  failed  in  the  race,  or  had  to  occupy  stations 


FORESTS   OF  NORTH  AMERICA. 


26 


of  inferior  advantage.  As  early  as  the  Carboniferous  period 
the  slender  trunk,  supporting  a  canopy  of  foliage  at  a  con- 
siderable height  above  the  ground,  showed  how  immediately 
the  needs  of  the  crowded  life  of  the  forest  had  been  met  by 
the  architecture  of  these  plants. 


Cycad  in  the  Botanical  Gardens,  Cape  Town,  South  Africa. 

The  trees  of  the  first  great  forests,  those  which  gave  us  the 
beds  of  peat  which,  in  time,  became  the  coals  of  the  Carbon- 
iferous period,  were  not  destined  to  endure  ;  they  were  weakly 
structures,  incapable  of  withstanding  cold,  and  demanding  a 
larger  share  of  moisture  than  could  be  afforded  outside  of  the 
limits  of  the  swamps  of  that  time.      Moreover,  their  seeds  were 


264  ASPECTS   OF  THE  EARTH. 

o-enerally  microscopic  in  size,  containing  none  of  that  nutri- 
ment which  enables  the  young  of  our  higher  plants  to  start  in 
the  race  of  life  with  a  share  of  sustenance  provided  by  the 
parent. 

It  is  true  that  we  do  not  know  with  any  certainty  what  was 
the  character  of  the  plant  life  which  during  the  Carboniferous 
period  occupied  the  uplands  of  the  lands.  The  coal-beds  pre- 
serve to  us  only  the  swamp  deposits  of  that  period.  From 
this  fact  some  students  of  the  coal-measures  have  concluded 
that  there  may  have  been  an  assemblage  of  more  highly 
organized  plants  on  the  dryer  portions  of  the  earth's  surface. 
It  is  possible  that  such  may  have  been  the  case  ;  but  among 
these  coal-measures  we  have  the  deltas  of  numerous  streams 
which  should  have  borne  down  from  the  upland  some  of  the 
drift-wood  from  the  plants  which  grew  there.  The  fact  that 
none  of  these  deposits  have  yielded  plants  of  a  higher  char- 
acter is  fair  evidence  that  the  whole  of  the  surface  of  the  dry 
land  was  covered  by  plants  of  very  inferior  organization. 

Already  in  the  Carboniferous  period,  and  in  the  Permian, 
we  begin  to  see  the  forerunners  of  a  higher  form  of  plants — 
forms  allied  to  our  living  conifers  and  yews  ;  they  were  rela- 
tively rare  forms,  yet  they  were  the  beginnings  of  a  higher 
order  of  life.  One  stage  higher  on  the  geological  section  these 
early  conifers  and  yews  are  re-enforced  by  other  large-seeded 
plants  of  the  same  group  akin  to  the  cypresses  and  the  cycads. 
But  the  greatest  advance  in  the  forests  consisted  in  the  intro- 
duction of  the  palms.  The  ferns  continue  to  be  an  important 
element  in  the  forests  ;  but  slowly  they  are  pushed  into  a  posi- 
tion of  inferiority,  their  places  being  gradually  taken  by  the 
higher  forms  of  cone-bearing  plants  and  cycads.  Lastly,  in 
the  relatively  recent  ages  of  the  later  Cretaceous  and  Tertiary 


FORESTS    OF  NORTH  AMERICA.  265 

there  came  the  higher  flowering  plants,  which  give  us  the  pre- 
vaiUno-  trees  of  our  modern  forests — oaks,  poplars,  elms,  and 
the  other  familiar  broad-leaved  plants,  which  generally  send 
down  their  leaves  during  the  period  of  winter  rest.  Owing  to 
their  many  advantages  of  structure  and  of  function,  these  last 
comers  are  steadfastly  gaining  the  room  which  once  belonged 
to  the  ancient  pines. 

The  broad-leaved  flowering  plants,  when  they  take  on  the 
tree  form,  manifest  their  superiority  in  many  ways  ;  besides 
their  larger  seeds,  which  give  some  of  the  parent's  strength 
to  aid  the  nursling  at  its  first  struggles  for  existence,  they 
have  a  better  framework  on  which  to  support  the  great 
association  of  buds  which  constitutes  the  tree.  Unlike  the 
first  trees,  which  generally  had  hollow  or  spongy  stems, 
which  did  not  suit  the  needs  of  large-branched  forms,  they 
have  dense  wood  in  the  centre,  which  admirably  serves  for 
the  support  of  the  colony  of  buds  and  permits  a  great  height 
of  the  trunk.  Thus,  while  the  largest  trees  of  the  coal  period 
probably  did  not  lift  their  branches  to  the  height  of  one 
hundred  feet,  many  of  the  forms  of  the  present  day  climb  for 
light  to  the  height  of  two  or  three  hundred  feet  above  the 
earth.  But  the  greatest  advantage  of  the  modern  trees  is 
probably  found  in  the  fact  that  they  often,  by  the  help  of 
insects  and  other  means,  secure  a  cross-fertilization  of  their 
flowers,  so  that  the  seeds  of  one  plant  are  fecundated  by  the 
pollen  of  another.  This  cross-fertilization  appears  to  give  to 
the  progeny  of  the  plant  a  better  chance  in  the  combat  for 
existence  than  they  can  secure  where  the  seeds  are  fertilized 
by  the  same  flower  or  those  of  the  same  colony  or  tree. 

Another  important  advance  which  has  been  made  in  the 
organization  of  trees  is  found  in   the  peculiar  order  in  which 


266 


ASPECTS   OF  THE  EARTH. 


the  buds,  and  consequently  the  branches,  are  placed  in  the 
association  of  separate  growth  centres  which  make  up  an 
ordinary  plant.  In  order  to  secure  something  like  an  even 
share  in  the  advantages  of  light  and  air  which  are  so  essential 
to  vegetable  growth,  these  buds  need  to  be  disposed  in  definite 


A  Group  of  Palms,    Bay  Biscoyne,  Florida. 


and  orderly  relation  to  each  other.  In  the  lowest  plants  this 
feature  of  organization  is  extremely  imperfect.  The  separate 
growth  centres  are  huddled  together  without  any  very  definite 
order.  Gradually  with  the  gain  of  experience  which  life  brings 
to  all  the  organic  scries,  the  centres  of  growth  are  brought  into 
definite  relation  the  one  to  the  other.  Thus  in  many  of  our 
plants  we  see  that  the  leaves  come  off  alternately,  sometimes 


FORESTS   OF  NORTH  AMERICA.  267 

in  the  form  of  pairs  arising  at  the  same  level,  followed  next 
above  by  another  pair  set  at  right  angles  to  the  preceding,  and 
with  a  third  pair  of  leaves  immediately  above  the  second.  In 
other  cases  the  third  pair  are  not  just  above  the  second,  but 
we  must  proceed  yet  higher  on  the  stem  before  we  find  the 
circuit  completed.  In  higher  plants  this  order  of  arrangement 
of  the  buds  frequently  becomes  extremely  complicated  ;  but  in 
all  cases  the  relative  position  of  the  buds  seems  to  be  generally 
ordered  so  as  to  secure  an  even,  if  rather  complicated,  relation 
of  the  growth  centres  one  to  another,  to  the  end  that  the  sun- 
shine may  visit  all  parts  alike.  This  feature  in  the  organiza- 
tion of  plants  is  slowly  evolved  during  the  successive  geologic 
periods,  and  a  final  success  in  the  arrangement  is  one  of  the 
peculiar  advantages  which  come  to  highly  organized  forms. 

The  result  of  these  improvements  is  that  the  struggle  is  at 
present  mainly  between  the  broad-leaved  trees  and  the  coni- 
fers. The  palms  survive  only  on  or  near  the  tropics,  and  the 
tree-ferns  remain  as  remnants  of  a  life  which,  once  of  supreme 
importance,  is  now  at  an  end.  Our  most  successful  forests  are 
those  of  the  broad-leaved  trees.  These  predominate  in  all  the 
great  forests  of  temperate  latitudes.  Their  variety  of  forms 
being  far  greater  than  those  of  the  conifers,  they  are  ready  to 
seize  on  any  station  which  the  chances  of  the  battle  afford 
them.  They  have  already,  to  a  great  extent,  driven  the  coni- 
fers to  the  more  northern  and  intemperate  stations,  or  to  the 
sandier  and  more  arid  soils  of  the  northern  hemisphere. 

With  this  inadequate,  though — we  may  hope,  from  the 
nature  of  the  subject — interesting,  glance  at  the  history  of  our 
forests,  let  us  go  to  some  tract  of  primeval  woods,  to  see  what 
are  the  conditions  of  the  land  beneath  its  mantle  of  vegetation. 
Let  us  take  a  district  where  broad-leaved  trees  predominate, 


268  ASPECTS    OF  THE  EARTH. 

for  there  the  characteristic  conditions  of  our  modern  forests 
are  best  displayed.  There  is  no  place  so  well  suited  for  this 
inquiry  as  the  field  of  the  great  Appalachian  forest,  which  lies 
on  the  uplands  of  the  region  within  a  radius  of,  say,  one  hun- 
dred miles  of  the  great  mountains  of  North  Carolina.  In  this 
area  are  still  to  be  found,  perhaps,  the  finest  areas  of  virgin 
woods  of  the  deciduous  type  that  remain  upon  the  earth.  The 
trees  are  of  exceeding  variety,  and  man  has  as  yet  spared  them 
the  destruction  which  he  is  soon  to  inflict. 

The  natural  entrance  to  these  forests — often,  indeed,  the 
only  practicable  way  into  their  recesses — is  up  the  channels  of 
the  streams.  Such  were  the  ways  by  which  the  early  settlers 
penetrated  the  wilderness  with  their  pack-trains  or  rude  wag- 
ons, and  they  still  afford  the  only  roads  to  many  of  the  settle- 
ments of  this  region.*  We  note  that  the  longer  streams  of 
this  wilderness,  those  deserving  the  name  of  rivers,  are  so  wide 
that  they  cut  a  channel  through  the  forest ;  but  from  the  allu- 
vial plain,  on  either  side,  white-trunked  sycamores  and  the  del- 
icately foliaged  willows  spring,  like  the  remains  of  old  arches, 
far  out  over  the  water.  As  the  stream  narrows,  so  that  its 
channel  is  not  more  than  fifty  feet  wide,  these  inclined  trees, 
on  either  side,  commingle  their  branches,  forming  an  arch  of 
interlaced  boughs.  We  note  the  crystal  purity  of  the  water 
contained  in  these  streams  ;  even  in  times  of  flood  it  contains 
but  little  waste  from  the  soil,  though  it  may  be  discolored  by 
the  stain  of  the  decayed  forest  bed  over  and  through  which  it 
has  passed.     Comparing  this  stream  of  pure  forest  water  with 

*We  can  still  trace  the  difficult  progress  of  those  modern  pilgrims  by  the  names 
they  gave  the  streams  up  which  they  toiled — Dismal  Creek,  Troublesome  Creek, 
Hell-for-certain  and  Pull-and-be-damned  Creeks,  and  yet  more  descriptive  names 
mark  the  stages  of  their  journey. 


FORESTS    OF  NORTH  AMERICA. 


269 


that  which  is  derived  from  a  valley  where  there  are  extensive 
tilled  fields,  we  see  one  of  the  most  striking  evidences  of  the 
evil  arising-  from  man's  presence.  In  such  a  stream  from 
ploughed  land  we  see,  after  every  rain,  that  the  water  is 
exceedingly  discolored  with  sediments,  and  that,  besides  the 
floatingr   mud,   a   larofe   amount  of  sand    is  driven    alone   the 


Stream  obstructed  by  Fallen  Timber. 


bottom  by  the  current.  The  mud  is  hurried  away  to  the 
lower  rivers,  and  thence  to  the  sea  ;  but  the  sand  and  pebbles 
gather  in  baj^s  which  hinder  the  course  of  the  stream,  com- 
pelling it  to  turn  about  in  a  devious  way,  cutting  into  its 
banks,  widening  its  bed,  and  destroying  its  former  beauty.  In 
times  of  flood  it  is  a  raging  torrent ;  in  periods  of  drought  it 
is  often  quite  dry.      Ascending  our  typical  stream  still  further, 


270 


ASPECTS    OF  THE  EARTH. 


to  where  its  diminished  waters 
are  only  a  score  or  so  of  feet 
wide,  we  find  its  course  embar- 
rassed by  many  fallen  trunks  of 
trees,  which  the  stream  has  not 
the    power  to    sweep  away  as 
it  does  in   the  wider  channels 
below.        Many     of     these 
fallen   trees    have   caught 
'     ^   the  smaller  drifting  frag- 
ments    of     wood,     the 
whole  forming  a  toler- 
ably tight  dam  which, 
for  a   time,   retains 
a    portion    of    the 
flood  waters,   al- 
lowing-    them 
gradually      to 
filter  through 
the     inter- 
stices,    thus 
partly    main- 
taining    the 
volume  of  the 
stream     in 
seasons   of 
drought. 

Turning 

from  the  way  of  the  stream  into  the  deep  shadow  of  the  forest 
which  bounds  it  on  either  side,  we  find  ourselves  at  once  in 
a  realm  unknown    to  ordinary  experience.      Even   in  winter, 


^^'''^^m 


Black-walnut,  Floyd  County,  Ky.     (Ky.  Geological  Survey.) 


FORESTS   OF  NORTH  AMERICA. 


271 


when  the  leaves 
are  shed,  the  close- 
set  branches  halve 
the  sun's  rays,  and 
in  sum  m  e  r  the 
brightest  sky  af- 
fords there  only  a 
gloaming  such  as 
we  see  in  the  open 
eround  after  sun- 
set.  Looking  up- 
ward, we  see  the 
trunks  rising,often 
without  a  limb, 
to  the  height  of 
more  than  one 
hundred  feet,  and 
to  the  surface  of 
the  crreat  domes 
of  foliage  it  is 
often  a  distance  of 
two  hundred  feet 
from  the  ground. 
The  constant 
struggle  for  light 
causes  every  space 
in   the   orreat  can- 

o 

opy  of  foliage  to 
be  filled  by  the  con- 
tending branches. 
The  surface  of  the 


Yellow-pine,  Harlan  County,  Ky.     (Ky.  Geological  Survey.) 


272  ASPECTS    OF  THE  EARTH. 

ground  is  thickly  covered  by  fallen  trunks  of  the  trees  which 
have  lived  their  term  of  life  and  returned  to  the  earth.  Some 
of  them  are  reduced  to  the  form  of  long,  low  mounds,  deeply 
covered  with  moss,  so  decayed  and  worm-eaten  that  the  foot 
sinks  in  them  as  into  snow, — others  still  keeping  the  sem- 
blance of  their  giant  forms,  even  in  their  prostrate  position. 
These  trees  have  rarely  been  overthrown  by  the  storms ; 
except  in  the  path  of  a  hurricane,  the  wind  is  unfelt  in  these 
shades  ;  they  fall  as  a  strong  man  by  a  sudden  blow.  Those 
who  are  accustomed  to  haunt  these  primeval  woods  have  often 
observed  how,  in  the  months  of  May  or  June,  when  the  air  is 
perfectly  quiet,  oftenest  in  the  dead  of  night,  while  the  woods 
are  as  still  as  a  cavern,  there  comes  through  the  silent  aisles  of 
the  forest  a  roar  as  of  far-off  thunder.  The  din  is  caused  by 
some  old  tree,  whose  trunk,  sapped  by  decay  and  overweighted 
by  the  burden  of  its  new-made  leaves  and  sap,  has  fallen  into 
ruin. 

The  tangle  of  decayed  vegetation  which  covers  the  ground 
beneath  the  forest  is  of  considerable  thickness.  On  top  it 
consists  altogether  of  the  decayed  trunks,  branches,  and  leaves, 
but  it  shades  downward  into  ordinary  dark-colored  soil  at  the 
depth  of  a  few  feet  from  the  surface.  This,  the  decay  zone  of 
the  forest,  lies  between  the  boughs  of  the  air  and  the  branches 
of  the  roots.  In  it  go  on  the  most  important  actions  which 
take  place  in  our  forests— actions  which  affect  the  history  of 
land  and  sea.  We  shall  therefore  have  to  consider  it  in  a 
somewhat  pains-taking  way.  The  most  evident  effect  of  this 
sheet  of  decaying  wood,  and  moss  which  feeds  on  the  decay,  is 
on  the  rainfall  of  the  region  which  it  mantles.  When,  after  a 
season  of  drought,  a  copious  rain  falls  upon  this  spongy  mass, 
the  water  is  for  a  long  time  absorbed  in  the  interstices,  and 


A   Tulip  Tree,    Bell    County,    Kentucky. 


FORESTS    OF  NORTH  AMERICA.  273 

does  not  flow  to  the  rivers.  Even  in  times  of  very  heavy  rain 
the  water  is  slowly  yielded  to  the  streams  ;  after  a  dry  period 
of  many  weeks  this  sponge  retains  a  good  share  of  water.  A 
like  amount  of  rain  falling  on  tilled  fields  or  prairies  slips 
quickly  away  to  the  rivers,  and  thence  to  the  sea.  The  first 
result  is,  that  when  the  land  is  destitute  of  forests  it  sheds 
water  like  house-roofs,  breeding  floods  after  every  considerable 
rain,  while  in  the  forests  the  rain  is  only  slowly  yielded  to  the 
streams. 

A  second  and  less  evident  result  of  the  spongy  character 
of  the  forest  bed  is  that,  by  hindering  the  escape  of  the  rain- 
water to  the  rivers,  it  increases  the  actual  rainfall  of  the 
country.  To  see  the  nature  and  importance  of  this  action, 
we  must  turn  aside  for  a  moment  to  consider  the  origin  of  the 
rain  which  falls  upon  the  land.  The  original  source  of  this 
water-supply  is  the  sea,  which  sends  into  the  lands  a  tolerably 
regular  annual  store  of  moisture.  When  this  falls  as  rain  or 
snow,  either  of  two  things  may  happen — the  water  may  go 
away  directly  to  the  sea,  or  it  may  return  to  the  atmosphere 
as  vapor  to  be  again  precipitated  as  rain.  The  chance  of  its 
re-evaporation  is  determined  by  the  speed  with  which  it  flows 
to  the  streams.  From  a  treeless  region  it  rapidly  escapes  ;  in 
an  extensive  district  of  virgin  forest  it  may  again  and  again 
pass  from  earth  to  air,  and  from  air  to  earth. 

The  columns  of  vapor,  which  in  times  of  summer  rain 
may  be  seen  ascending  from  every  great  wood,  afford  visible 
evidence  of  the  effect  of  forests  on  rainfall.  They  also  may 
show  the  observer  some  of  the  most  beautiful  phenomena  of 
atmospheric  circulation.  In  a  summer  rain-shower  the  air 
above  the  trees  becomes  much  cooler  than  it  is  in  the  recesses 
below  their  tents  of  foliage.     This  heated  air  within  the  wood 


2  74  ASPECTS   OF  THE  EARTH. 

seeks  to  rise,  and  escapes  in  great  columns  wherever  there  is 
a  wide  gap  between  the  branches  ;  as  soon  as  it  attains  the 
cooler  level  above,  the  moisture  is  condensed,  and  the  air, 
before  transparent,  becomes  charged  with  steam.  To  replace 
this  ascending  air,  a  broad  current  drifts  toward  the  emerging 
streams  of  vapor,  commonly  from  the  higher  parts  of  the 
forest,  where  the  air,  owing  to  the  elevation  of  the  site,  is 
cooler  than  in  the  lower  levels. 

This  repeated  passage  of  the  moisture  from  earth  to  cloud, 
and  from  cloud  to  earth,  greatly  increases  the  amount  of  force 
which  the  rain  applies,  in  its  falling  drops,  to  the  earth's  sur- 
face ;  but  the  rank  vegetation  protects  the  surface  from  the 
erosion  which  it  would  otherwise  bring  about.  Even  the 
forest-clad  hill-sides  of  the  Cumberland  Mountains,  where 
the  soil  lies  on  declivities  of  great  steepness,  suffer  little  wear 
as  long  as  their  natural  protection  is  left  to  them.  But  as 
soon  as  they  are  stripped  of  the  garment  of  wood  which  has 
been  upon  the  region  ever  since,  in  the  far-off  ages,  they 
came  from  the  depths  of  the  sea,  they  wear  with  great  rapid- 
ity. The  erosion  is  limited,  as  long  as  they  are  forest-clad, 
to  the  stream  beds,  and  there  is  hindered  by  the  innumerable 
obstacles  of  the  fallen  trees  and  entangled  driftwood.  The 
brooks  which  are  strong  enough  to  clear  their  beds,  and  cut 
into  the  earth  and  rock,  are  few  in  number  ;  we  may  often,  on 
the  flatter  ground,  find  tracts  of  a  square  mile  or  more  in  area 
in  which  there  is  not  a  single  stream  that  ever  assails  the 
surface  of  the  earth.  As  soon,  however,  as  the  forest  mat 
is  removed,  the  surface  becomes  seamed  with  channels  ;  they 
often,  on  the  deforested  surface,  increase  one-hundred-fold  in 
their  length,  and  more  than  that  measure  in  their  destructive 
power.      Relieved  of  all  restraint  from  fallen  timber,   or  the 


FORESTS   OF  XORTH  AMERICA.  275 

close-knit  roots  which  enmesh  the  earth,  they  sweep  the  pre- 
cious soil  away  toward  the  sea.  In  a  single  day  a  tilled  field 
may  lose  from  its  surface  more  soil  than  would  be  taken  from 
it  in  a  century  of  its  forest  state. 

It  is  in  this  action  of  the  rain  upon  the  bared  surface  of 
the  ground  that  we  find  the  principal  danger  which  menaces 
man  in  his  use  of  the  earth.  The  forests  probably  take  each 
year  from  the  soil  as  much  as  our  tilled  crops  ;  but  they  not 
only  retain,  in  the  ash  of  the  decayed  leaves,  branches,  and 
trunks,  all  which  they  have  removed,  but  they  allow  little 
waste  to  occur  through  the  action  of  rain  and  wind.  The  use 
which  man  makes  of  the  soil,  when  he  tills  it,  is  almost  neces- 
sarily destructive,  not  only  through  the  harvests  which  he 
removes,  but  by  the  incidental  waste  which  occurs  in  the  soil 
which  is  washed  or  blown  away  to  the  sea.  We  behold  the 
results  of  this  perilous  wasting  in  every  country  which  has 
long  been  the  seat  of  tillage.  In  level  regions  it  is  least 
apparent,  but  in  all  hill  countries  it  is  quickly  and  often 
deplorably  manifested.  The  destruction  in  the  United  States 
is  most  serious  in  the  northern  tier  of  Southern  States,  but 
no  portion  of  the  tilled  districts  is  quite  exempt  from  it. 
Brief  as  has  been  our  use  of  this  American  land,  a  perceptible 
portion  of  it,  probably  as  much  as  one-hundredth  part  of  the 
tillable  area,  has  been  reduced  to  a  state  of  destitution  which 
it  will  require  ages  to  repair — which,  indeed,  is  scarcely  repar- 
able by  the  hand  of  man. 

Turning  from  the  general  aspects  of  the  forest  to  the 
details  of  its  organization,  we  should  first  notice  the  great 
range  in  the  character  of  its  individual  occupants,  the  various 
species  of  trees  which  form  the  wood.  To  the  variety  of  the 
kinds  of  trees  which  are  associated  together  the  Appalachian 


2/6  ASPECTS    OF  THE  EARTH. 

forest  owes,  in  good  part,  the  wonderful  success  which  it  has 
attained.  The  coniferous  woods  of  this  region  rarely  have 
more  than  five  or  six  species  to  share  the  possibilities  of  a 
given  field ;  but  in  the  broad-leaved  forests  we  often  find  no 
fewer  than  fifty  species,  each  of  which,  in  a  similarly  extensive 
area,  finds  a  place  suited  to  the  peculiar  capabilities  to  which 
it  has  attained.  In  this  element  of  variety  our  American  for- 
ests far  exceed  those  of  Europe  which,  in  a  general  way,  they 
closely  resemble.  The  deciduous  woods  of  the  old  world 
have  not  more  than  one-fourth  as  many  species  of  trees  as  we 
find  in  those  of  Eastern  North  America.  For  instance,  in 
North  America  we  have  thirty  or  more  species  of  oak,  to  the 
three  or  four  of  Europe,  and  many  genera,  including  some  of 
the  noblest  forms — such  as  the  tulip  trees,  the  magnolias,  the 
gums,  and  the  swamp-cypress — which  have  no  representatives 
in  the  present  European  forests. 

The  advantages  arising  from  this  great  diversity  of  species 
in  our  American  deciduous  woods  are  easily  conceived.  Each 
of  these  kinds  has  developed  a  special  adaptation  to  some  par- 
ticular conditions  of  soil,  moisture,  or  exposure  ;  so  that  every 
opportunity  which  the  conditions  afford  is  met  by  some  par- 
ticular kind  of  tree,  each  making  haste  to  avail  itself  of  all 
the  chances  which  are  afforded  to  it.  Thus,  in  the  Southern 
swamps  the  Taxodium,  or  bald  cypress,  has,  by  a  very  singu- 
lar arrangement  of  its  roots,  succeeded  in  adapting  itself  to 
soils  which  are  permanently  covered  with  water — a  chance 
which  is  denied  to  other  laree  trees.  From  each  root  which 
extends  beneath  the  swamp  about  the  trunk  there  arise  spurs. 
These  spurs  grow  upward  in  the  form  of  stout  columns,  each 
capped  by  a  bud-like  excrescence,  hollow  in  the  centre,  and 
covered  externally  by  a  spongy  bark,  through  which  the  sap 


FORESTS   OF  NORTH  AMERICA. 


277 


circulates.  These  bulbous  or  bud-like  knobs  are  often  so 
large  and  hollow  that  they  are  sometimes  cut  off  and  used 
by  the  farmers  in  the  swamp  districts  for  bee-hives  and  well- 
buckets.     The  height  above  the  surface  of  these  "  knees,"  as 


The  Swamp-cypress  (Tennessee),  showing  the  Spurs. 


the  projections  are  called,  is  so  adjusted  that  in  the  growing 
season  their  upper  parts  are  above  the  level  of  the  water;  if 
by  any  chance,  as  when  a  mill-dam  has  raised  the  level  of  the 
pool,  the  tops  of  these  appendages  of  the  roots  are  covered 
during  the  spring  and  early  summer  seasons,  the  tree  inevit- 
ably dies.      So  immediately  is  this  contrivance  fitted  to  the 


278  ASPECTS   OF  THE  EARTH. 

needs  of  the  situation  that  a  cypress  tree  on  the  border  of  a 
swamp  will  have  the  knees  on  those  roots  which  extend  beneath 
the  water,  while  those  which  run  under  the  higher  ground  will 
fail  to  produce  them.  When  the  tree  is  artificially  grown  on 
hio-h  ground,  the  knees,  so  far  as  observed  by  the  present 
writer,  are  never  developed;  or,  at  most,  remain  as  trifling 
spurs  on  the  roots,  which  do  not  rise  above  the  soil.  Thus 
the  bald  cypress,  though  quite  unable  to  contend  with  the 
deciduous  trees  of  the  dry  forests,  has  a  safe  stronghold  in 
the  vast  swamps  of  the  Southern  States,  and  forms  some  of 
the  noblest  wood  of  this  country. 

The  sour  gum,  another  of  the  swamp-dwelling  trees, 
though  not  so  successful  in  such  positions  as  the  cypress,  has 
a  contrivance  for  exposing  the  roots  to  the  air  much  like  that 
which  we  find  in  the  bald  cypress  in  its  effects,  though  the 
structure  is  differently  devised.  In  place  of  sending  a  spur 
from  the  root,  the  root  itself  arches  upward  in  a  horseshoe 
form,  so  that  the  top  of  the  bend  rises  above  the  plane  of 
the  water  during  the  growing  season.  Both  this  arrange- 
ment of  the  tupelo  and  that  of  the  cypress  probably  afford 
to  the  tree  an  opportunity  to  bring  the  sap  of  the  roots  in 
contact  with  the  air  before  it  is  drawn  into  the  bole  of  the 
tree. 

The  willows,  the  cottonwoods,  and  the  sycamores,  also, 
find  a  special  field  in  immediate  contact  with  the  water, 
though  they  have  no  such  provision  as  the  cypress  for  dwell 
ing  in  the  permanently  inundated  ground.  They  commonly 
live  on  the  banks  of  the  rivers,  and  feed  on  the  fertile  soil 
which  the  inundations  bring  to  the  shores.  Leaning  their 
trunks  toward  the  streams,  and  expanding  their  branches  in 
the  open  space  above  them,    they,    like  the  cypresses,  win  a 


FORESTS    OF  NORTH  AMERICA.  279 

realm  where  they  do  not  have  to  contend  with  their  stouter 
competitors  for  light  and  air. 

The  alluvial  lands  on  either  side  of  the  streams,  regions 
liable  to  frequent  floods,  are  possessed  by  species  which  have 
a  less  endurance  of  humidity  than  the  forms  just  mentioned, 
but  are  still  more  tolerant  of  long-continued  floods  than  the 
most  of  our  deciduous  trees.  Here  we  find,  especially  in  the 
southern  part  of  the  Appalachian  forest,  pin-oaks,  certain 
kinds  of  elms,  the  swamp  chestnut-oaks,  gums,  tulip  trees, 
etc.  So  completely  is  the  forest  adjusted  to  the  conditions, 
that  the  alluvial  lands  of  the  rivers  gienerally  bear  a  different 
assemblage  of  trees  from  those  of  the  smaller  streams. 

In  the  upland  districts  the  trees  are  distributed  in  a  more 
varied  manner  than  in  the  parts  of  the  surface  which  are 
affected  by  inundations ;  still,  even  there  the  arrangement  is 
rather  accordino^  to  evident  law  than  accordingf  to  the  indis- 
cernible  complexity  of  law  we  term  chance.  Though  the 
species  are  somewhat  affected  by  the  accidents  which  plant 
the  seeds,  the  predominance  of  any  species  is  always  indica- 
tive of  some  peculiarities  of  soil  or  exposure.  So  accurate  is 
this  delimitation  that  the  early  settlers  in  the  forested  Western 
States  always  and  unerringly  chose  their  places  of  settlement 
by  the  nature  of  the  timber.  Where  blue-ash,  black-walnut, 
or  coffee  trees  abounded,  they  knew  that  they  had  the  most 
fertile  soils  ;  beech  woods  indicated  a  soil  of  less  fertility,  but 
still  of  endurance  to  tillage  ;  white-oaks  a  soil  of  lower  grade, 
but  suited  to  certain  crops — as,  for  instance,  to  tobacco  ;  a 
predominance  of  red-oak  trees,  a  yet  less  suitable  ground  ;  and 
a' wood  of  black-oaks,  or  "black-jacks,"  the  most  unpromising 
field  of  all. 

In  a  hilly  country  each  of  the  varying  aspects  of  the  sur 


28o 


ASPECTS   OF  THE  EARTH. 


face  brings  its  peculiar  influences  to  bear  on  the  distribution 
of  the  timber.  So  sharply  is  the  distribution  determined  by 
the  compass  direction  of  the  slope,  and  the  consequent  share 
of  sunshine  which  it  affords,  that  a  skilled  observer  may,  in 
cloudy  weather,  tell  the  direction  of  his  way  by  a  careful  study 
of  the  forest. 

We    should    also    observe  that  the    same    immediate  and 


\i:^m. 


Black-jack  Oaks,  Todd  County,   Ky.     (Ky.  Geological  Survey  ) 

complete  adaptation  of  the  trees  to  their  conditions  is  shown 
in  the  way  in  which  they  recover  their  possession  of  aban- 
doned fields,  and  of  the  tracts  which  have  been  deforested 
by  hurricanes  or  fire.  A  certain  limited  number  of  species 
lead  the  way  to  the  re-possession  of  these  districts  from  which 
the  forest  has  been  expelled.  If  the  ground  has  been  long 
under  tillage,  as  many  of  the  worn-out  fields  of  Virginia  have 
been,  the  sassafras,  persimmon,  black-  and  red-oak  are  apt  to 


FORESTS   OF  NORTH  AMERICA.  281 

be  the  first  of  the  forest  trees  to  estabHsh  themselves.  If  fire 
has  done  the  work,  then  the  poplars  and  birches  have,  in  most 
districts,  the  best  chance  ;  if  it  be  a  hurricane's  path,  the 
ground  is  sure  to  be  full  of  seeds  and  young  trees,  and  it  is 
any  one's  race,  with  the  predominant  oaks  generally  in  the 
lead.  South  of  Virginia,  where  the  soil  is  sandy,  the  "old- 
field  pine  "  is  generally  the  pioneer  of  the  forest's  re-advance. 

Nowhere  is  the  process  by  which  the  forests  recover  the 
possession  of  fields  from  which  they  h^ve  been  driven  by  the 
plough  more  victoriously  shown  than  in  New  England.  In  that 
region  the  skirts  of  every  wood  are  bordered  by  a  thick-set 
undergrowth  largely  composed  of  our  bayberry  and  huckle- 
berry bushes,  which  rapidly  restore  to  the  soil  the  surface 
coating  of  mould  necessary  to  the  best  growth  of  larger 
plants.  The  nutritious  berries  are  eaten  by  the  birds,  and 
the  seed  scattered  over  the  fields,  so  that  in  a  few  years  after 
tillage  has  ceased  little  mounds  of  foliacre  are  scattered  here 
and  there  over  the  open  ground.  Each  of  these  colonies  of 
fruit-bearing  plants  becomes  a  natural  cradle  for  the  nurture 
of  any  chance  seed  of  larger  trees  which  may  be  wafted  to  the 
sheltered  station  they  afford. 

The  controlling  conditions  in  the  distribution  of  the  forest 
trees  are  :  first,  the  characteristics  of  the  species  ;  second,  the 
nature  of  the  soil  ;  third,  the  chance  of  distribution  of  the 
seed  ;  and,  lastly,  the  assaults  of  the  animal  enemies  which 
each  kind  encounters.  Some  species — as,  for  instance,  the 
black-locust — are  extensively  subjected  to  insect  enemies. 
The  oaks,  on  the  other  hand,  are,  on  account  of  their  acrid 
sap,  tolerably  well  protected  from  such  dangers.  As  a  whole, 
our  deciduous  trees  have  established,  by  one  device  and 
another,  a  tolerably  strong  defence  against  animal  pests.      In 


282  ASPECTS   OF  THE  EARTH. 

fact,  they  owe  their  continued  existence  to  their  success  in 
preventing  this  class  of  dangers.  Now  and  then  some  new 
enemy  arises,  which  imperils  and  may  destroy  a  species.  An 
instance  of  this  sort  has  recently  come  to  the  attention  of  the 
present  writer  from  some  study  of  the  forests  of  Kentucky, 
which  was  undertaken  with  the  co-operation  of  his  assistants. 
It  appears  from  these  observations  that  the  white-oaks  of  that 
district,  which,  despite  the  ravages  of  the  axe,  still  constitute 
some  of  its  finest  forests,  are  in  the  way  to  disappear,  owing 
to  their  failure  to  reproduce  their  kind.  There  are  singularly 
few  young  white-oaks  in  these  woods,  but  an  abundance  of 
the  less  desirable  varieties  of  red-  and  Spanish-oaks.  The 
reason  for  this  seems  to  be  that  the  nuts  of  the  white-oak 
are  more  palatable  to  the  squirrels  than  those  of  the  other 
species  ;  so  these  creatures  industriously  seek  them  out,  and 
only  resort  to  the  more  bitter  and  probably  less  nutritious 
nuts  of  the  other  species  when  those  of  the  white-oak  fail 
them.  In  similar  ways  other  animals  react  destructively  upon 
other  forest  trees.  The  introduction  of  swine  in  the  settled 
portions  of  the  forests  brings  a  greedy  and  judicious  palate  to 
consume  the  more  edible  nuts,  and  so  destroy  the  progeny  of 
many  trees.  But  what  is  one  kind's  loss  is  another  kind's 
gain  ;  with  the  destruction  of  one  species,  its  competitors  find 
a  fair  field  and  hasten  to  occupy  it. 

There  are  evidently  two  principal  limiting  causes  which 
determine  the  growth  of  forests — these  are  drought  and  cold. 
When  the  rainfall  is  less  than  serves  to  keep  the  roots  moist 
during  the  period  of  growth,  or  when  the  growing  season  is 
too  brief  to  permit  the  ripening  of  the  new  wood  of  a  tree, 
the  forests  find  their  limit.  In  the  struggle  with  the  cold  the 
coniferous  trees  have,  in  general,  the  advantage  of  the  broad- 


FORESTS   OF  NORTH  AMERICA.  2d>2) 

leaved  group,  possibly  for  the  reason  that  in  the  former  class 
a  portion  of  the  foliage  holds  over  the  winter  ;  thus  there  is 
less  to  do  to  bring  the  machinery  of  growth  into  operation, 
and  the  process  of  annual  increase  can  be  more  quickly  accom- 
plished. So,  too,  in  the  struggle  with  arid  conditions  the 
conifers,  or  narrow-leaved  trees,  appear  to  be,  on  the  whole, 
more  successful  than  the  broad-leaved  trees,  probably  for  the 
reason  that  their  rigid  and  scanty  foliage  expends  less  water 
than  the  soft  and  expanded  leaves  of  the  other  group.  Thus 
the  conifers  have  come  to  occupy  the  greater  part  of  the 
scantily  watered  districts  of  the  Rocky  Mountains,  as  well 
as  the  regions  of  the  far  north  up  to  the  limits  of  the  forests 
which  stretch  toward  the  North  Pole. 

We  have  already  noticed  the  process  by  which  forests 
recover  their  possession  of  ground  from  which  they  had  been 
driven  by  the  destructive  work  of  tillage.  From  time  to  time 
the  return  of  these  native  woods  to  the  fields  from  which 
they  have  been  dispossessed  by  natural  processes  occurs  in 
a  majestic  way.  The  successive  glacial  periods,  which  from 
time  to  time  in  geological  history  have  swept  the  high  lati- 
tudes of  the  several  continents,  destroy  not  only  the  forests 
but  the  soils  on  which  they  are  fed  in  a  wide-spread  way. 
Thus  after  the  last  ice  time,  when  with  the  climatal  change 
the  glacier  disappeared,  the  northern  part  of  North  America 
— in  general  the  country  north  of  the  parallel  of  40 — was  a 
waste  of  sand,  gravel,  and  other  detrital  materials,  containing 
no  trace  of  the  old  soil.  As  the  glaciers  disappeared  this  vast 
area  was  a  free  field  open  to  the  forest  species  which  had  been 
driven  southward  before  the  advance  of  the  ice-sheet.  Into 
this  area  the  plants  made  haste  to  enter.  The  small-seeded 
trees,   such  as  the  willows,   which    have    eerms    that    can    be 


2  84 


ASPECTS   OF  THE  EARTH. 


readily  carried  by  the  wind,  were  fleetest-footed  and  had  the 
choice  of  their  appropriate  stations.  Other  trees,  such  as  the 
maple,  the  seeds  of  which,  though  heavy,  have  parachute-like 
wings  which  may  carry  them  far,  move  also  with  a  certain  speed 
towards  their  goal.  The  heavy-seeded  trees,  such  as  the  wal- 
nuts, the  hickories,  beeches  and  oaks,  are  compelled  to  travel 

more  slowly  ;  the  wind 
has  little  effect  on  their 
seeds ;  they  trust  for 
transmission     to     the 


Winged  Elm  (showing  foliage  on  the  edge  of  a  forest),  Cumberland  Valley,  Ky. 

short  advance  made  each  generation  by  the  length  to  which 
the  boughs  spread  from  the  parent  trunk,  or  they  secure  a 
wider  scattering  by  squirrels  or  other  small  animals  which  eat 
the  nuts  and  are  thus  led  to  carry  them  about  either  in  their 
jaws  or  stomachs.  The  result  is  that  the  heavy-seeded  trees 
have  lagged  behind  in  the  race  for  high  latitudes,  while  the 
light-seeded  firs,  willows,  and  other  plants  which  are  easily 
dispersed  have  won  the  greater  part  of  the  fields  left  vacant 


FORESTS   OF  NORTH  AMERICA.  285 

by  the  retreat  of  the  glaciers.  We  perceive  by  the  fact  that 
they  will  grow  much  further  to  the  north  than  the  stations 
^vhich  they  now  occupy,  that  the  large-seeded  plants  are  hob- 
bled in  the  race  for  high  latitudes,  while  the  light-seeded 
plants  have  generally  attained  as  high  latitude  as  they  can 
possibly  withstand. 

In  their  race  for  the  high  north  the  forest  trees  are  clearly 
subject  to  the  influence  of  selection  in  such  a  way  as  to  deter- 
mine the  death  or  life  of  many  forms  ;  so,  too,  when  driven 
southward  by  the  ice-sheet  the  process  of  automatic  choice, 
which  we  term  the  survival  of  the  fittest,  occurred.  When 
the  ice-sheets  appear  in  high  latitudes  and  thence  move  south- 
ward, each  species  must  migrate  or  perish  ;  the  individual 
forms  of  course  are  not  free  to  move,  but  the  succession  of 
;generations  must  win  their  way  southward  with  sufficient 
speed  to  keep  ahead  of  the  on-coming  ice.  If  any  species 
failed  in  this  work  it  would  inevitably  be  overwhelmed  by 
the  glacier,  and  thus  disappear  from  the  face  of  the  earth.  In 
the  return  to  the  regions  made  vacant  by  the  disappearance 
of  the  ice,  the  process  of  selection  would  be  accomplished  in  a 
different  way,  but  to  the  same  effect.  The  species  which  did 
not  move  northward  fast  enough  would  find  the  climate  change 
more  rapidly  than  their  dwelling-place,  and  so,  if  they  did  not 
reconcile  themselves  to  the  new  kind  of  weather  they  would 
perish,  or  they  might  be  overwhelmed  by  other  and  stronger 
species  which  marched  far  into  their  territory.  Thus  the  fleet- 
footed  forms  would  always  have,  other  things  being  equal,  an 
advantage  in  the  race  for  life.  But  "other  things"  are  not 
commonly  equal  in  the  work  of  nature  ;  while  the  large-seeded 
forms  are  slower  to  make  their  great  migrational  journeys, 
the  very  largeness  of    their  seeds  provides  them  with   more 


286  ASPECTS   OF  THE  EARTH. 

sustenance,  fits  them  better  for  the  struggle  for  life  which 
comes  in  infancy.  Thus,  though  the  heavy-seeded  trees  are 
plodding  back  into  the  old  glaciated  fields  slowly,  they  are, 
nevertheless,  moving  with  a  stubborn  determination  which  bids 
fair  to  give  them  possession  of  all  those  wide  realms. 

It  is  an  interesting  fact,  which  we  had  occasion  to  notice  a 
few  pages  back,  that  the  forest  trees  of  Europe  are  of  much 
less  variety  in  their  kinds  than  those  of  North  America.  The 
reason  seems  to  be  that  the  glacial  periods  in  Europe  serve 
to  overwhelm  the  vegetable  life,  and  this  because  when  the 
glacial  envelope  comes  upon  the  continent  and  forces  the  army 
of  plants  down  to  the  southward,  they  have  no  secure  field  for 
retreat,  as  in  North  America,  but  find  their  migrations  stopped 
by  the  great  gulf  of  the  Mediterranean.  It  is  likely  that  the 
wide  difference  between  the  richness  of  the  forest  life  in  the 
Old  World  and  the  New  is,  in  part  at  least,  determined  by 
this  cause. 

There  remains  still  the  need  of  explaining  the  absence  of 
forests  from  that  part  of  the  so-called  prairie  districts  of  the 
West  where  the  water-supply  is  abundant  for  the  nurture  of 
forests,  and  where,  indeed,  a  great  variety  of  native  forest 
trees  grow,  when  planted,  with  singular  luxuriance.  Several 
recondite  explanations  have  been  devised  to  account  for  this 
peculiarity.  It  has  been  asserted  that  these  prairies  are  unfa- 
vorable to  the  growth  of  trees  because  of  the  fineness  of  the 
soil  ;  but  without  considering  how  this  Ji?ieitcss  can  act,  it  is 
easy  to  see  that  the  soil  of  the  Mississippi  delta  region,  which 
intersects  the  prairie  districts,  is  even  finer-grained  than  that 
of  the  treeless  plains.  Others  have  held  that  these  regions 
were  the  floors  of  great  lakes,  which,  after  the  glacial  period, 
were    quickly  possessed    by  grasses  to  the  exclusion  of    the 


FORESTS    OF  NORJH  AMERICA. 


287 


trees.      But  it  is  easy  to  see  that,   even    in  the   best  of   our 
existing  grass  lands,  the  forests  generally  manage  swiftly  to 


A  Sycamore  Tree  in  White  River  Bottoms,  near  Wheatland,  Ind. 

regain   possession  when  they  are  allowed  to  pursue  their  way 
without  interference  from  man. 

In  accounting  for  the  prairies,  it  will  not  do  to  seek  their 
origin  in  a  single  cause.  There  are  certainly  two  elements 
at  least  in  the  causation  which  have  operated  with  different 


288  ASPECTS   OF  THE  EARTH. 

degrees  of  effect  in  different  parts  of  the  Western  tracts.  In 
the  region  beyond  the  Mississippi,  where  the  annual  rainfall  is 
less  in  amount  than  twenty  inches,  drought  alone  will  perhaps 
serve  to  explain  the  treeless  conditions  ;  farther  to  the  east  an 
artificial  cause — viz.,  the  fires  whicli  the  Indians  were  in  the 
habit  of  setting  to  the  grass  of  the  open  ground  and  the  leaves 
of  the  woods — will  account  for  the  destruction  of  the  original 
forests.  These  annual  forest  fires  were  kindled  either  to  drive 
the  game  toward  the  hunters  or  to  aid  the  growth  of  the  fresh 
grass  which  springs  up  after  the  conflagration.  In  this  way 
the  prairies  were  extended  eastward  to  Indiana  and  south  to 
the  Ohio  River.  At  a  point  west  of  Louisville,  Ky.,  the 
prairie  country  crossed  that  stream,  and  extended  south  to  the 
Cumberland  River,  near  where  Nashville  now  lies.  In  this 
latter  region  we  have  a  clear  example  of  tlie  process  by  which 
the  country  was  deforested.  When  the  whites  first  came  to 
the  Ohio  valley,  this  prairie  region  between  the  Ohio  and  the 
Cumberland  Rivers  occupied  the  whole  belt  of  limestone  land 
of  Western  Kentucky.  Skirting  the  southern  border  of  the 
western  coal  field,  it  extended  westward  across  the  Cumber- 
land and  Tennessee  Rivers  into  the  low  table-land  which  lies 
between  the  last-named  stream  and  the  Mississippi  River. 
About  five  thousand  square  miles  of  this  area  were  actually 
deforested,  except  where,  beside  the  scanty  streams,  the  ground 
was  too  moist  to  permit  the  ravages  of  the  annual  conflagra- 
tions. On  the  border  of  this  area  the  old  trees  were  not  de- 
stroyed, but  remained  in  the  form  of  a  very  open  forest.  The 
younger  growth  was,  however,  wanting.  The  reason  for  this 
is  plain  :  The  older  trees  have  a  very  thick  outer  bark,  which 
served  to  protect  them  from  the  damage  which  would  be  in- 
flicted by  the  momentary  heat  of  the  burning  leaves,  while  the 


FORES  IS   OF  NORTH  AMERICA. 


289 


tender  stems  of  the  saplings  were  easily  destroyed.  Thus  it 
came  about  that  when  the  old  trees  died  they  left  no  successors, 
and  so  the  prairie  steadily  widened  its  area. 

As   soon   as   the    Indians   ceased   to   use   Kentucky  as  an 
annual  hunting-ground,  the  forests  rapidly  regained  their  pos- 


session  of   all    the   prairie 

lands  of  this  district.    The    PIS^ 

annual  burning  of  the  sur-  nsX^v.-4«s» /| 

face       ceased      in       the      latter  Ash  Grove,  Ashland,   Fayette  county,   Ky. 

part  of  the  last  century  ;  in  the  second  decade  of  this,  the 
whole  of  this  great  area  was  covered  by  a  thin  wood  of  young 
trees,  which  quickly  closed  into  a  dense  forest.  At  the  present 
time  all  the  parts  of  this  field  which  have  not  been  deforested 
by  man  are  thickly  wooded.  Some  indications  of  a  similar 
process  of  forest  restoration  may  be  found  in  Indiana  and 
Illinois  ;  but  in  those  regions  the  annual  rainfall  is  less,  and 


290  ASPECTS    OF   THE  EAR'IH. 

summer  droughts,  which  are  calculated  to  prevent  the  establish- 
ment of  the  young  trees,  are  more  frequent  and  more  pro- 
longed than  in  Kentucky. 

Turning  now  to  consider  the  underground  work  of  the 
forests,  we  find  there  a  realm  of  activities  of  interest  equal  to 
that  of  their  more  visible  portion.  The  leaf-bearing  branches 
of  the  trunk  are  hardly  more  extensive  than  those  which 
penetrate  the  soil.  The  main  function  of  these  underground 
branches  is  to  supply  the  ashy  element  of  the  plant,  which  they 
take,  dissolved  in  water,  from  the  soil,  and,  in  the  form  of  sap, 
send  upward  to  the  leaves  for  further  elaboration.  In  this 
work  they  penetrate,  not  only  horizontally  through  the  exist- 
ing soil  bed,  but  often  enter  into  the  crevices  of  the  rocks 
which  have  not  yet  been  converted  into  earthy  matter.  As 
soon  as  the  roots  find  a  profitable  way  into  these  fissures  of 
the  rocks  beneath  the  soil,  they  increase  in  size  and  exert  a 
powerful  rending  action,  riving  the  stones  asunder.  Each 
weak  place  of  the  fragments  is  in  turn  sought  out,  and  the 
hard  mass  is  in  time  reduced  to  small  bits  as  effectively  as  by 
the  blows  of  a  hammer.  This  work  often  goes  on  at  a  depth 
of  ten  feet  or  more  below  the  surface  ;  and  so  forest  trees  oper- 
ate to  produce  soils  of  great  depth,  while  the  grasses  and  tilled 
crops  have  much  less  effect. 

The  greatest  work  of  the  forest  on  the  subjacent  earth  is 
accomplished  by  the  action  of  the  deep  layer  of  decaying 
vegetation  which  it  forms  upon  the  surface.  This  layer  con- 
sists mainly  of  carbon,  which,  by  the  process  of  decay,  is 
combined  with  the  oxygen  of  the  air  in  the  proportion  of  two 
atoms  of  the  latter  to  one  of  the  former  substance.  This 
combination  is  known  to  chemists  as  CO^  or  carbonic  diox- 
ide ;  or,  in  the  old  nomenclature,  as  carbonic-acid  gas.     It  is, 


FORESTS   OF  NORTH  AMERICA.  291 

as  the  old  name  Indicates,  a  gas  ;  and  though  heavier  than 
air,  in  good  part  escapes  into  the  atmosphere  in  time  to  be 
reclaimed  by  the  leaves  and  to  return  to  some  forest  bed. 
This  gas  is  extremely  soluble  in  rain-water,  each  part  of  water 
taking  in  many  times  its  bulk  of  the  gas.  At  first  sight  this 
seems  a  very  commonplace  fact  ;  but,  as  we  shall  see,  on  it 
depends,  in  an  intimate  way,  a  most  important  part  of  the 
mechanism  of  the  earth. 

When  the  water  falls  on  the  surface  of  the  earth  it  has 
little  pbwer  to  take  into  solution  the  substances  which  com- 
pose the  rocks.  The  charge  of  carbonic-dioxide  gas  increases 
this  dissolving  capacity  in  a  wonderful  manner  ;  for  instance, 
when  pure  rain-water  will  dissolve  one  portion  of  lime,  the 
same  water  when  charged  with  the  gas  will  take  fifty  times  as 
much  into  solution.  So  with  nearly  all  the  substances  which 
the  water  encounters  ;  its  solvent  influence  is  vastly  increased 
by  the  carbonic  dioxide  contributed  by  the  forest  bed.  Along 
Avith  this  eras  the  forest  bed  adds  to  the  water  a  number  of 
other  acids  derived  from  the  decaying  vegetation,  all  of  which 
serve  in  different  degrees  to  promote  its  solvent  action. 

The  most  immediate  effect  of  this  action  is  to  enable  the 
roots  to  appropriate  the  mineral  matters  of  the  soil,  which 
they  cannot  seize  on  until  they  are  brought  into  solution. 
Thus  the  dead  plants  serve  the  functions  of  the  living  in  a 
most  important  way.  But  it  is  in  the  remoter  effects  of  these 
carbonated  waters  that  we  discover  their  most  important  role. 
Soaking  deep  into  the  earth,  they  find  their  way  slowly  into 
the  interstices  of  the  rocks,  and  take  from  them  something  of 
their  contents.  When,  after  a  long  journey  in  the  under- 
ground, they  emerge  in  the  springs,  they  bear  away  to  the 
sea  a  share  of  about  all  the  substances  which  are  found  in  the 


292  ASPECTS   OF  THE  EARTH. 

crust  of  the  earth.  The  ability  of  water  to  carry  away  these 
materials  is  mainly  due  to  the  influences  exerted  by  the  thick 
coat  of  decaying  vegetation  through  which  it  passed  on  enter- 
ing the  earth. 

The  contribution  which  these  spring-waters  make  to  the 
sea  provides  it  with  that  wide  range  of  dissolved  materials 
which  is  necessary  for  the  sustenance  of  marine  life,  and  for 
the  formation  of  the  deposits  composed  of  organic  remains. 
The  mud  and  sand  which  is  carried  by  the  rivers  to  the  ocean 
has  relatively  little  value  as  an  agent  in  such  formation,  for 
the  reason  that  it  is  all  deposited  near  the  mouths  of  the 
streams.  If  this  work  of  undero-round  water  were  not  done, 
or  even  were  it  done  with  half  its  present  efficiency,  the 
oceans  would,  when  their  present  store  of  dissolved  mineral 
matters  was  exhausted,  cease  to  maintain  their  vigorous  crea- 
tive work. 

We  thus  see  that  the  soil  coatino;  of  the  earth's  surface, 
which  is,  in  the  main,  the  product  of  forest  action,  is  a  neces- 
sary part  of  the  machinery  that  fits  both  sea  and  land  for  the 
uses  of  organic  life.  In  the  vast  enginery  of  the  earth,  where 
there  are  so  many  parts  absolutely  necessary  for  the  work  of 
supporting  the  functions  of  the  whole,  it  is  hardly  possible  to 
speak  of  any  one  contrivance  as  of  pre-eminent  importance  ; 
still  this  complicated  work  of  the  forests  may  fairly  be  con- 
sidered as  of  critical  value  to  the  interests  of  life. 

The  foregoing  considerations,  though  all  too  brief  for  our 
need,  will  enable  us  to  consider  the  economic  aspects  of  our 
American  forests  in  a  summary  wa)^  It  is  clear,  from  what 
has  been  said,  that  the  most  important  aspect  of  the  problem 
concerns  the  soils.  The  great  question  is.  What  will  be  their 
fate    in    the    deforested    condition    into  which   they  must  be 


FORESTS   OF  NORTH  AMERICA.  293 

brought  for  the  chief  uses  of  man  ?  It  is  clear  that  in  all 
countries  the  waste  arising  from  the  erosive  action  of  the 
rain  is  far  crreater  in  tilled  grround  than  in  forest-clad  dis- 
tricts.  Indeed,  in  forests  we  may  say  that  the  soil  is  ever 
deepening,  while  in  tilled  lands  it  is  almost  always  diminish- 
ing in  depth.  There  are  certain  conditions  in  this  country 
which  make  the  rate  of  wear  more  rapid  than  on  the  Euro- 
pean continent.  The  rainfall  of  the  district  east  of  the  Mis- 
sissippi is  greater  and  more  torrential  in  its  character,  and 
therefore  more  erosive,  than  in  the  old  world.  Therefore 
we  may  expect  danger  from  this  cause  in  much  less  time  than 
it  has  been  encountered  in  other  countries. 

It  is  evident  that,  as  far  as  this  evil  is  a  necessary  accom- 
paniment of  tillage,  it  must  be  borne  as  best  it  may  ;  but  in 
large  part  it  is  capable  of  correction  by  the  exercise  of  a  little 
intelligence.  As  the  amount  of  this  erosion  is,  in  general, 
directly  proportional  to  the  steepness  of  slope  of  the  ground, 
abrupt  declivities  should  not  be  subjected  to  the  plough,  but 
retained  in  timber  or  in  grass.  If  tilled  at  all  they  should  be 
terraced,  as  is  now  much  of  the  steeper  ground  of  Europe. 

The  next  danger  is  that  which  arises  from  the  sudden  pre- 
cipitation of  the  rainfall  into  the  streams  when  the  forests  are 
cleared  away.  This  process  brings  about  serious  inundations 
in  the  season  of  rain,  followed  in  the  times  of  drought  by  a 
drying  up  of  the  streams  which  were  once,  by  the  action  of 
the  forest,  maintained  throughout  the  year  with  a  more  equal 
flow.  This  evil  is  already  manifested  in  the  condition  of  the 
Western  rivers.  With  the  removal  of  the  forests,  the  winter 
floods  increase  in  magnitude,  and  the  summer  droughts  leave 
so  little  water  in  the  streams  that  they  are  constantly  becom- 
ing less  serviceable  for  navigation.      Moreover,  the  amount  of 


294  ASPECTS    OF  THE  EARTH. 

soil  which  is  swept  into  the  rivers  is  so  great  that  they  are 
embarrassed  by  it  ;  their  channels  are  shallowed,  and  the 
currents,  driven  to  and  fro,  widen  the  water-way,  and  thereby 
shallow  the  diminished  streams  in  the  season  of  low  water. 

In  part  this  evil  is.  like  the  tirst  mentioned,  inevitable, 
for  it  is  due  to  man's  necessary  interference  with  the  forest 
covering  of  the  earth  ;  still  it  may  be  minimized.  The  law 
has  interfered  to  prevent  the  owners  of  the  Californian 
placers  from  pouring  the  waste  of  their  hydraulic  washings 
into  the  streams,  because  they  harm  those  who  live  and  labor 
on  the  banks  below  ;  on  the  same  principle,  we  may  fairly 
require  the  tiller  of  the  soil  to  keep  the  soil  of  his  fields  where 
it  belongs,  by  adapting  his  treatment  of  the  ground  to  the 
limitations  which  its  nature  imposes  upon  him.  When  the 
present  crude  notions  of  the  rights  of  the  owners  of  land 
have  become  qualified  by  reason,  when  it  is  accepted  that  the 
possessor  of  land  has  only  a  reasonable  usufruct  in  the  piece 
of  the  earth  of  which  he  holds,  and  that  he  has  no  right  to 
use  it  wastefully  or  to  his  neighbor's  injury,  we  may  meet  this 
problem  with  fair  success. 

It  seems  to  the  present  writer  that  the  government  has  a 
right  to  require  that  all  the  existing  forests,  the  preservation 
of  which  may  be  deemed  necessary  to  the  good  of  the  val- 
ley in  which  they  lie,  should  be  maintained  in  their  present 
condition  ;  or,  if  removed,  that  they  should  be  replaced  by 
equivalent  plantations  of  timber.  This  can  be  so  managed 
that  the  owners  shall  retain  all  that  these  woods  have  of 
present  value — viz.,  the  timber,  as  it  ripens,  for  exportation. 
The  owners  may  lose  an  "  unearned  increment "  of  prospec- 
tive value  of  these  lands  for  tilled  fields  and  town  sites, 
but    concerning  this  justice  need  not  be    seriously  troubled. 


FORESTS    OF  NORTH  AMERICA.  295 

So  rapid,  indeed,  is  the  appreciation  on  the  value  of  forest 
products,  that  the  restriction  would  bring  little  that  can  be 
called  hardship  to  the  owners  of  these  forests.  The  damaore 
already  done  to  our  rivers  by  the  removal  of  forests  is  not  so 
great  that  it  cannot  be  borne  ;  moreover,  it  can  be  in  good 
part  compensated  by  a  proper  system  of  reservoirs,  in  which  a 
portion  of  the  winter  flood-water  may  be  retained  until  the 
times  of  summer  drought." 

The  next  disadvantage  arising  from  the  removal  of  the 
forests  is  due  to  the  loss  of  the  secondar}^  rainfall,  or  that 
arising  from  the  evaporation  of  the  moisture  retained  by  the 
spong\''  bed  and  embarrassed  streams  of  the  primeval  woods. 
Fortunately,  this  country  has  in  the  most  of  its  originally 
forested  regions  a  greater  surplusage  of  annual  rainfall  than 
have  most  of  the  other  civilized  districts  of  the  world,  and, 
therefore,  can  better  afford  to  lose  the  valuable  aid  of  occa- 
sional showers  such  as  this  evaporation  induces.  ^Moreover, 
the  rapid  extension  of  irrigation,  which  is  sure  to  take  place 
in  the  more  arid  sections  of  the  country,  will  afford  a  similar 
and,  perhaps,  in  time  an  equal  supply  of  moisture  for  these 
secondary  rains. 

We  come  now  to  the  uses  of  forests  as  sources  of  timber- 
supply.  From  this  point  of  view  we  find  their  most  imme- 
diate and  unquestionable  value  to  man.  The  ages  of  stone, 
bronze  and  iron  have  succeeded  each  other  in  the  arts,  but 
through  them  all  man  has  always  been  a  wood-using  animal. 
Only  the  beaver  approaches  him  as  a  consumer  of  timber. 
While  the  general  substitution  on  the  hearth  of  coal — the 
product   of    ancient  forests — for   the  timber    from   the  living 

*See,  for  further  consideration  of  this  point,  The  Floods  of  the  Mississippi  Valley, 
Atlantic  Monthlj-,  vol.  li.,  p.  697. 


296  ASPECTS   OF  THE  EARTH. 

woods — has  diminished  one  element  of  man's  ravage,  the 
development  of  modern  society  steadfastly  increases  the  tax 
which  each  individual  levies  on  the  forest.  Although  some 
dreamers  conceive  that  in  the  future  man  may  make  use  of 
aluminum  as  a  substitute  for  wood,  there  is  no  reason  to 
believe  that  this  change  will  ever  be  accomplished.  So  far 
each  addition  of  cheaper  metal  has  served  to  increase  rather 
than  to  diminish  the  demand  on  our  timber-resources,  and 
this  is  likely  to  be  the  case  in  all  the  foreseeable  future. 

The  general  abandonment  of  wood  as  a  fuel  has,  however, 
changed,  in  an  important  way,  the  nature  of  the  drain  upon 
our  forests.  For  firewood,  the  forest  is  cleared  away ;  for 
construction-timber,  the  natural  growth  is  not  usually  de- 
stroyed— only  the  ripe  trees  need  be  removed,  leaving  the 
physical  character  and  influence  of  the  forest  essentially  un- 
changed. Where,  as  in  Germany,  the  forests  are  generally 
plantations  owned  by  private  citizens  or  by  the  govern- 
ment, the  whole  field  is,  it  is  true,  cleared  away  at  once, 
but  the  laws  require  that  before  each  clearing  is  effected  a 
similar  area  shall  be  freshly  planted  ;  thus  the  forest  is  kept 
intact. 

The  present  condition  of  our  American  forests,  except 
those  of  the  cordilleran  region,  is  by  a  fortunate  combination 
of  accidents  in  a  much  more  satisfactory  shape  than  might 
have  been  expected  from  the  rapid  growth  of  our  population. 
In  the  first  place,  the  head-waters  of  the  streams  of  the  east- 
ern part  of  the  continent  lie  generally  in  the  rugged  hills  of 
the  Appalachian  Mountains,  where  the  soil  is  not  the  most 
fertile  and,  therefore,  the  inducement  to  tillage  slight.  On 
this  account  the  most  important  forests  for  their  effect  on 
rainfall  and  on  the  water-supply  of  our  streams  often  remain 


FORESTS    OF  NORTH  AMERICA.  297 

in  a  comparatively  little-changed  state.  Next,  the  agricul- 
tural districts  of  the  Southern  States,  where  a  great  deal  of 
the  old  forest  has  been  cleared  away,  are  regions  of  large  rain- 
fall, and  generally  of  tolerably  level  surface,  so  that  neither 
the  evils  of  desiccation  nor  those  arising^  from  the  washing  of 
the  soil  to  the  sea  have  as  yet  proved  very  serious.  Still 
further,  the  wide  prairie  lands  of  the  Mississippi  valley  have 
taken  into  their  forestless  areas  nearly  one-third  of  the  soil- 
tillers  of  our  population,  and  so  have  given  us  a  field  of 
expansion  without  much  immediate  effect  on  the  area  of  for- 
ests. The  timber-supply  for  these  prairie  States  has  largely 
come  from  the  region  about  the  great  lakes,  and  its  removal 
has  therefore  had  little  effect  on  navigable  streams  or  on  the 
summer  rains. 

Lastly,  the  abundant  and  cheap  transportation  of  food- 
products  from  these  prairies  to  the  Appalachian  district  and 
the  Atlantic  shore  lands  rescued  their  forests  from  the  axe  by 
making  the  tillage  of  all  but  the  most  fertile  lands  unprofit- 
able. The  effect  of  this  Western  food-supply  has  been  so 
great  that,  since  the  middle  of  the  century,  it  has  not  only  in 
good  part  stopped  the  process  of  clearing  away  the  Appa- 
lachian woods,  but  in  certain  districts,  especially  in  New 
EnMand,  extensive  areas  of  land  which  had  been  lontr  under 
tillage  have  been  allowed  to  return  to  the  forest  state.  In 
Massachusetts,  for  instance,  it  is  probable  that  the  area  now 
possessed  by  timber  is  considerably  greater  than  it  was  at 
the  beginning  of  the  present  century.  The  present  writer, 
in  his  many  journeys  through  this  State,  has  observed  many 
thousand  acres  of  woods  where  the  marks  of  former  tillage — 
corn-hills,  and  walls  composed  of  bowlders  gathered  from  the 
once  cleared  land — attest  the  sometime  clearing  away  of  the 


298  ASPECTS   OF  THE  EARTH. 

forest ;  but  he  has  not,  in  all,  seen  as  much  as  one  thousand 
acres  which  had  recently  been  won  to  the  plough. 

Thus  the  matter  of  forest  preservation  is,  in  the  main,  a 
problem  for  the  immediate  future  :  save  in  the  valley  of  the 
Ohio  and,  perhaps,  on  the  Adirondacks,  the  process  of  de- 
struction has  not  as  yet  begun  to  give  extremely  serious 
results.*  It  seems,  however,  certain  that  the  conditions  which 
have  postponed  the  question  of  forest  management  have 
about  exhausted  their  influence,  and  that  in  the  next  half- 
century  we  shall  imperatively  need  a  systematic  control  of 
our  remaining  timbered  districts. 

The  Western  prairie  lands,  or  at  least  those  parts  of  that 
vast  region  of  treeless  plains  which  are  suitable  for  tillage 
without  a  costly  system  of  irrigation,  are  generally  possessed 
by  settlers.  Already  the  population  begins  to  press  back 
upon  the  older  districts,  which  were  passed  by  as  long  as  the 
best  lands  of  the  West  were  to  be  had  for  the  asking.  A 
large  part  of  the  Appalachian  forest  district,  though  affording 
poorer  soils  than  the  prairies,  is  second-class  land  which  can 
be  profitably  tilled  if  we  count  as  profit  the  interest  of  the 
farmer  alone,  without  considering  the  effect  of  this  tillage  on 

*  The  destruction  of  forests  in  the  cordilleran  region  has  been  much  more  seri- 
ous than  that  in  the  eastern  portion  of  the  continent,  though  it  has,  in  the  main, 
been  accomplished  by  fire,  not  by  the  axe.  The  loss  of  these  Rocky  Mountain  for- 
ests is  especially  to  be  regretted,  for  the  reason  that  the  deforested  areas,  owing  to 
the  prevailing  dryness  of  the  climate,  do  not  spontaneously  return  to  the  timbered 
condition  as  they  do  in  the  regions  east  of  the  Mississi]5pi.  It  is  a  rude  compensa- 
tion for  the  loss,  that  this  destruction  only  deepens  the  already  hopeless  sterility 
which  marks  the  greater  part  of  the  mountain  region  of  the  West.  On  the  Pacific 
coast  there  are,  as  is  well  known,  superb  forests  of  coniferous  woods  which  are  in 
much  danger  of  ruin.  For  many  reasons,  however,  these  forests  are  of  less  critical 
importance  than  those  of  the  eastern  district.  They  do  not  greatly  affect  the  regi- 
men of  important  streams,  and  are  almost  witliout  influence  on  the  rainfall. 


FORESTS   OF  NORTH  AMERICA.  299 

the  rest  of  the  country.  Considered  from  a  wider  point  of 
view,  we  cannot  afford  to  have  it  tilled  ;  for  the  reason  that 
we  need  the  existing  forests  for  the  supply  of  timber  they 
may  afford,  as  well  as  for  their  effect  on  the  rainfall  and  the 
water-supply  of  the  rivers  in  whose  valleys  they  lie. 

The  forests  are  a  precious  heritage  of  man.  They  pro- 
vided him  a  cradle;  they  furnished  him  the  soil,  and  they 
still  offer  him  their  help  in  some  of  his  greatest  needs.  No 
man  has  the  right  to  destroy  them  when  their  destruction 
means  calamity  to  his  fellows  or  his  successors.  To  give  the 
individual  the  right  to  appropriate  and  overthrow  them  at  his 
will  is  to  constitute  him  a  cruel  despot ;  if  such  privileges 
exist  in  the  laws  framed  by  a  short-sighted  past,  it  is  time 
they  were  annulled. 


THE   ORIGIN  AND   NATURE    OF    SOILS. 


Relation  of  Soils  to  Organic  Life. — Origin  of  Soils  ;  Effects  of  Solar  Heat ;  Influence  of 
Atmosphere. — How  to  Begin  the  Study  of  Soils. — Stages  of  Soil  Formation  ;  Action  of 
Rain  ;  of  Frost. — Effect  of  Lichens  ;  of  Higher  Plants. — Study  of  Mountain  District. 
— Effect  of  Joints  ;  of  Roots  of  Trees. — Processes  of  Torrent  Valleys  ;  Floods  ;  Rock 
Avalanches. — Diablerets  ;  Goldan  ;  Yvorgne  ;  White  Mountains. — Alluvial  Plains  ;  Pro- 
cesses of  Formation  of  ;  Soils  of. — Upland  Soils  ;  Immediate  Derivation  of  ;  Structure  of; 
Processes  of  Formation  of. — Process  of  Ablation. — Glacial  Soils  ;  Conditions  of  Conti- 
nental Glaciers  ;  Nature  of  Glacial  Soils  ;  Origin  of  Fertility  of  Soils  ;  Phosphate  Mat- 
ter :  its  Origin. — Effects  of  Penetrating  Air;  Means  whereby  Air  enters  the  Soil  — 
Effects  of  Tillage  ;  Action  of  the  Plough. — Proportion  of  Plant  Food  in  Soil. — Effect  of 
Ferments. — Comparison  of  Natural  and  Artificial  Conditions  of  Soil. — Working  of  Soils 
under  Tillage. — Man's  Duty  by  the  Soil. — Effects  of  Irrigation. 

The  relation  of  organic  life  to  the  earth  is  so  familiar  that 
it  does  not  seem  curious.  We  are  accustomed  to  see  the 
plants  and  animals  on  the  earth's  surface  ;  they  appear  to  be- 
long there,  and  we  rarely  note  the  very  peculiar  way  in  which 
they  manage  to  draw  their  sustenance  from  the  under  world. 
When,  however,  we  narrowly  examine  the  circumstances 
which  make  it  possible  for  organisms  to  be  nourished  by  the 
earth,  we  find  that  this  work  is  accomplished  by  a  very  elabo- 
rate mechanism  w^hich  depends  for  its  successful  operation 
on  extremely  complicated  conditions.  In  the  following  pages 
I  propose  to  set  forth  in  a  general  way  the  history  of  that 
superficial  part  of  the  land  surface  which  we  term  soils,  to 
show  how  it  has  come  into  existence,  and  by  what  means  it  is 
made  to  afford  a  medium  of  communication  between  the  so- 
called  dead  matter  of  the  earth  and  living  beings. 


THE  ORIGIN  AND  NATURE  OF  SOILS.  301 

To  understand  the  origin  of  soils  we  must  begin  by  study- 
ing the  earth's  surface  in  a  large  way.  It  is  necessary  to  note 
at  the  outset  that  the  soils  are  produced  not  by  forces  resident 
in  the  earth  itself,  but  in  those  which  come  to  it  from  the 
celestial  spaces.  Left  to  itself,  the  earth  would  have  presented 
no  trace  of  soil.  If  it  had  remained  in  the  condition  in  which 
it  was  when  it  cooled  down  from  its  original  molten  state,  its 
superficial  parts  would  have  had  the  character  of  lava  streams 
when  they  have  just  become  solid.  It  is  to  the  heat  which 
comes  to  our  earth's  surface  mainly  from  the  sun,  but  in  a  cer- 
tain measure  from  all  the  fixed  stars  as  well,  that  we  owe  the 
existence  of  this  medium  between  the  inorganic  and  the  organic 
world  which  we  find  in  our  soils.  This  heat  furnishes  the 
motive  power  by  which  the  rocks  are  pulverized.  Water  and 
the  atmosphere,  the  two  great  instable  envelopes  of  the  earth's 
surface,  afford  the  machinery  of  the  mill.  This  mill  produces 
a  layer  of  comminuted  rocky  matter  more  or  less  commingled 
with  organic  waste,  in  which  plants  may  find  a  place  for  their 
roots  ;  whence  they  may  draw  the  material  for  their  bodies. 
This  material  is  contributed  by  plants  to  animals,  and  affords 
the  opportunities  of  life  for  those  higher  organic  forms. 

The  best  way  to  begin  the  study  of  the  soil  is  to  observe 
what  takes  place  when  a  bare  surface  of  rock  is  laid  upon  the 
land  as  by  a  stream  of  lava,  or  in  some  newly-made  island  of 
the  sea;  or,  to  take  a  more  familiar  case,  we  may  examine  the 
exposed  rock  of  some  old  quarry  where  a  considerable  area 
has  been  deprived  of  its  soil  covering  and  left  slowly  to  re- 
cover this  coating  of  detrital  material.  Nearly  every  part  of 
the  civilized  world  will  afford  the  student  such  opportuni- 
ties for  observing  the  nature  of  the  process  by  which  soils 
are  made.      Even  where  quarries    do  not  exist,  an  old  pave- 


302  ASPECTS   OF  THE  EARTH. 

ment  will  in  many  cases  show  some  part  of  this  interesting 
process. 

The  stages  of  this  process  of  soil-making  are  as  follows  : 
The  sun's  heat  warming  the  waters  of  the  earth's  surface 
evaporates  them  and  lifts  them  high  into  the  atmosphere,  then 
they  fall  back  in  the  form  of  rain  or  snow.  Falling  in  the  form 
of  snow  the  water  has  the  least  influence  in  making  soils  ;  in 
the  form  of  rain  its  effects  are  more  immediate  and  important. 
The  snowflakes  come  down  with  no  perceptible  force,  but  the 
drops  of  water,  as  we  may  observe  by  holding  the  bare  hand 
exposed  in  a  heavy  shower,  strike  a  certain  blow  on  the  sur- 
face. Their  stroke  tends  to  loosen  any  grains  of  sand  or  mud 
which  may  be  by  decay  partly  severed  from  the  rock.  The 
result  is  that  in  a  short  time  a  considerable  amount  of  finely- 
divided  dcbi'is  is  accumulated  on  the  surface,  gathered  in  the 
little  hollows  which  are  always  found  there.  The  water  soaks 
into  the  interstices  of  the  stone ;  it  dissolves  the  material 
which  binds  the  grains  together,  and  so  prepares  them  to  give 
way  before  the  blows  which  subsequent  showers  bring  upon 
them.  Where  the  frost  operates  on  the  surface  by  expanding, 
the  water  which  has  been  absorbed  by  the  stone  still  further 
serves  to  break  up  its  structure.  In  the  quarries  of  granitic 
rock  which  abound  in  eastern  New  England,  a  few  years  of 
exposure  so  far  soften  the  stone  that  certain  lowly  forms  of 
plants  may  become  attached  to  it. 

The  first  forms  to  seize  upon  a  surface  are  lichens,  plants 
which  have  no  distinct  roots,  but  which  absorb  a  certain  por- 
tion of  mineral  matter  by  the  broad  adhesions  which  bind 
them  to  the  place  upon  which  they  fasten.  These  plants  are 
so  organized  that  they  may  remain  entirely  dry  for  a  good 
part  of  the  year,  becoming  active  only  at  those  times  when 


THE   ORIGIN  AND  NATURE  OF  SOILS.  303 

the  moisture  of  the  air  and  that  of  the  surface  on  which  they 
rest  afford  them  sufficient  water  for  their  vital  functions.  As 
soon  as  the  surface  has  become,  in  a  measure,  covered  with 
hchens,  the  decay  of  their  forms  suppHes  the  water  with  a 
certain  amount  of  carbonic-acid  gas,  which  vastly  enhances  its 
power  of  disintegrating  the  stone.  Ordinary  water,  as  it  falls 
in  the  form  of  rain,  can  dissolve  not  more  than  one  fifty-thou- 
sandth part  of  its  bulk  of  limey  carbonate,  or  ordinary  marble, 
while  if  charged  with  carbonic  acid  it  will  dissolve  one-thou- 
sandth part  of  its  mass  of  that  substance.  Thus,  as  soon  as 
vegetation  finds  a  foothold  on  a  bare  surface,  it  vastly  en- 
hances the  rate  at  which  the  air  can  proceed  in  forming  soils, 
for  in  proportion  as  the  solvent  power  of  the  water  is  increased 
its  effect  in  disintecrratinor  the  rock  is  increased. 

As  soon  as  the  surface  has  secured  a  coating  of  lichens 
the  process  of  soil-making  goes  on  with  increased  rapidity ; 
the  movinof  grains  of  sand  released  from  the  bed-rock  become 
entangled  between  the  broad  fronds  of  the  lichen,  and  are  no 
longer  washed  away  by  the  heavier  rains.  In  a  short  time 
enough  rocky  matter  is  accumulated  to  afford  a  place  in 
which  the  higher  plants,  those  bearing  roots,  may  find  a  foot- 
hold. They  grow  swiftly  in  the  short  period  when  the  thin 
soil  remains  moist,  and  when  dying  contribute  their  remains  to 
the  developing  soil.  In  the  course  of  a  century,  if  the  slope 
of  rock  be  not  too  steep,  a  thin  coating  of  debi'-is  is  spread 
over  its  entire  surface,  and  may  afford  sustenance  not  only  to 
mosses  and  lichens  but  also  to  plants  which  maintain  their 
life  during  the  whole  summer.  In  another  hundred  years 
herbaceous  plants  may  find  the  soil  thick  enough  for  their 
needs,  and  even  the  smaller  trees,  those  which  do  not  require 
to  strike  their  roots  into  a  deep  layer  of   earth,  may  fasten 


304  ASPECTS   OF  THE  EARTH. 

themselves  upon  it.  In  this  way,  in  a  period  which  is  but  an 
instant  of  time  measured  on  the  term  of  geological  ages,  an 
area  of  rock  which  has  been  by  accident,  as  by  an  earth- 
quake shock,  deprived  of  its  soil-coating,  or  on  which  that 
coating  has  never  previously  existed,  may  be  formed  ;  and  so 
the  region  is  brought  again  into  the  realm  of  living  beings. 

Althoutrh  this  small  illustration  serves  to  show  the  more 
general  facts  concerning  the  formation  of  soil,  there  is  very 
much  concerning  their  history  which  remains  to  be  told. 
Like  all  other  illustrations  drawn  from  limited  area  it  only 
aids  the   mind   to  conceive   the   larger  operations    of  nature 


'"// 


<iiill§li,i>  '■ 


Diagram  Showing  Conditions  of  Torrent  Valley  in  which  Alluvial  Soils  are  Fornning. 
A,  A,  Bed-rock.     B,  B,  Detritus  slowly  creeping  towards  stream.     C,  Torrent  bed. 

when  we  come  to  study  them  in  an  ampler  field.  The  prin- 
cipal difficulty  arising  from  such  an  illustration  is  that  it 
leaves  out  of  mind  all  circumstances  of  time  which  have  to 
be  considered  when  we  endeavor  to  follow  the  great  lines  of 
the  earth's  history.  The  soils  of  the  earth's  surface  must  be 
regarded  as  those  portions  of  its  hard  rock  which  are  on  their 
way,  after  undergoing  a  certain  amount  of  destructive  action, 
back  into  the  sea  whence  they  came,  and  where  they  are  in 
turn  to  be  employed  once  again  in  constructive  work. 

To  see  this  process  in  a  large  way  the  reader  should 
betake  himself,  at  least  in  imagination,  to  the  upper  portions 
of  a  considerable  mountain  district  on  which  the  work  of  soil- 


THE  ORIGIN  AND  NATURE  OF  SOILS.  305 

making  is  going  on  with  great  rapidity,  though  the  product 
of  the  work  may  not  remain  upon  their  steep  and  storm- 
swept  surfaces.  On  rocky  declivities  we  find  a  number  of 
agents  at  work  tending  to  disrupt  the  solid  earth.  We  have 
already  noted  the  fact  which  arises  from  the  slight  impact 
of  the  rain-drop.  This  effect  is  trifling  in  any  shower, 
but  accumulated  through  the  ages  it  becomes  an  agent  of 
considerable  importance.  Along  with  the  rain  comes  the 
lightning,  which,  striking  upon  the  dry  rocks  of  the  mountain- 
peaks,  often  effects  a  considerable  rending  of  their  masses. 
Here,  too,  gravity  operates  more  effectively  than  it  does  on 
the  lower  lands  where  the  surface  of  the  bed-rock  is  nearly 
level.  Gravitation  itself  does  nothing  to  promote  disinte- 
gration as  long  as  the  surface  is  level,  but  as  the  slopes  of 
the  hills  increase,  the  effect  of  this  force  augments  in  a 
very  rapid  manner.  When  the  slopes  exceed  forty  degrees 
in  declivity,  some  of  the  dislocated  fragments  are  almost  cer- 
tain to  roll  down  them  in  a  violent  way  and  to  be  broken  by 
the  shocks  which  they  receive  in  their  downward  movement. 
Wherever  the  bare  rocks  of  steep  mountain-sides  are  exposed, 
they  are  assailed  by  frosts.  All  rocks  whatsoever  are  pene- 
trated by  lines  of  fracture  which  are  termed  joints.  Every 
quarry  exhibits  these  joints,  indeed  it  is  only  where  a  rock 
is  advantageously  jointed  that  it  can  be  used  at  all  by  the 
quarrymen.  These  joints  are  in  their  original  form  only  incipi- 
ent lines  of  fracture ;  they  scarcely  appear  to  the  eye  before 
that  peculiar  attraction  known  as  capillarity  draws  water  with 
great  energy  into  them.  A  familiar  illustration  of  this  capil- 
lary action  is  seen  in  cases  where  dry  wood  becomes  wet  and 
expands  with  great  violence.  It  is  possible  to  split  stones  by 
driving  wooden  wedges  into  drilled  crevices  and  wetting  them 


306  ASPECTS   OF  THE  EARTH. 

with  water.  The  curious  tension  which  may  be  put  upon  a 
rope  by  wetting  it  is  due  to  the  energy  with  which  the  water 
is  drawn  into  the  spaces  between  its  fibres.  This  capillarity 
in  itself  serves,  in  a  measure,  to  rend  the  rocks,  but  it  works 
more  efficiently  in  this  direction  by  drawing  water  between 
the  plates  of  the  rock,  where,  when  it  becomes  frozen,  it  may 
expand  and  operate,  within  narrow  limits,  with  all  the  energy 
of  fired  gunpowder.  Each  winter's  freezing  pushing  the  plates 
of  the  rocks  a  little  distance  apart,  small  wedges  of  the  ma- 
terial also  loosened  by  frost  drop  into  the  crevice,  and  when 
the  ice  which  led  to  the  expansion  melts  away,  the  sides  of 
the  opening  can  no  longer  come  together.  The  next  winter 
this  is  repeated,  and  so  in  the  course  of  time  the  mass  is 
urged  beyond  its  supports  and  descends  as  a  rock  avalanche. 
These  joint  lines  are  also  the  channels  by  which  ordinary 
chemical  decay  penetrates  into  the  rock.  The  water  affords 
oxygen  which  by  promoting  decomposition  leads  to  the  break- 
ing up  of  the  stone.  If  there  be  iron  in  the  rock  in  the  form 
of  pyrite  or  magnetite,  it  may  become  oxidized  and  by  its  ex- 
pansion produce  a  thrust  tending  to  disrupt  the  masses.  If 
there  be  felspar  in  the  mass,  this  may  be  converted  into  kao- 
line  and  also  expand  by  the  change.  There  are  many  other 
actions  brought  about  by  chemical  decay,  operating  to  deprive 
the  rock  of  its  coherence  and  to  push  it  out  of  its  original 
resting-place. 

The  roots  of  trees  are  also  agents  of  great  power  acting 
with  all  the  energy  of  wedges  to  drive  the  stones  asunder. 
Again,  when  trees  are  overturned,  these  roots  which  have 
penetrated  between  the  crevices  of  the  stones  often  lift  consid- 
erable masses  out  of  their  beds,  and  place  them  in  positions 
where  when  the  enveloping  roots  decay  they  will  be  free  to 


THE   ORIGIN  AND  NATURE  OF  SOILS.  307 

tumble  down  the  slope.  When  steep  mountain-sides  are 
shaken  by  a  violent  earthquake,  great  masses  are  often  de- 
tached from  their  sides  and  descend  as  avalanches  to  the 
lower  lands.  Again,  when  the  mountains  are  snow-covered 
as  most  mountains  are  in  winter  time,  the  incoherent  snow 
often  precipitates  itself  into  the  valleys  below.  At  its  source 
the  avalanche  is  of  a  trifling  nature  ;  the  snow  light  as  feathers 
starts  down  the  slope  ;  when  it  has  moved  a  little  ways,  the 
pressure  of  the  part  which  is  in  motion  upon  the  lower-lyino- 
snow  causes  the  mass  to  become  compact,  and  in  a  short  time 
the  avalanche  is  more  like  a  mass  of  white  ice  than  the  snow 
in  which  its  movement  originated.  Gaining  depth  and  momen- 
tum as  it  descends,  the  stream  soon  begins  to  rend  away  the 
loose  materials  of  the  rock  surface  over  which  it  pours,  and 
in  the  course  of  half  a  mile  of  journey  it  often  comes  about 
that  a  quarter  or  more  of  the  avalanche  is  made  up  of  stones 
and  mud.  The  frozen  water  in  time  melts  away  from  the  heap 
which  has  been  precipitated  into  the  valley,  but  the  rocky 
matter  remains  to  be  dealt  with  by  the  stream  of  the  torrent- 
bed  in  which  it  lies. 

The  most  evident  effect  which  the  observer  perceives 
when  he  studies  the  above  noted  processes  is  that  stones 
are  thus  brought  down  from  their  lodgments  in  the  cliffs  and 
on  the  steep  slopes  of  the  mountains,  to  the  beds  of  the 
torrents  (see  chapter  on  Rivers  and  Valleys,  page  143).  A 
very  little  observation  will  show  him  that  in  the  torrent- 
bed,  the  rocks  are  rapidly  broken  to  pieces  by  being  driven 
against  each  other  and  pounded  against  the  sides  of  the 
mountain  as  the  brook  hurries  them  downward  to  the  low- 
lands. There  is  a  less  evident  but  still  important  influence, 
which  is  brought  about  by  this  movement  of  rocks  down  the 


3o8  A  SPEC  IS   OF  THE  EARTH. 

mountain-side.  If  we  take  a  pebble  or  any  other  mass  of  rock 
which  has  long  lain  upon  the  surface  of  the  earth,  we  per- 
ceive that  it  has  decayed  on  its  outer  surface  :  even  if  the 
decay  be  not  evident  to  the  naked  eye,  we  can  see  it  by 
scratching  the  surface  with  a  knife  ;  breaking  the  pebble  we 
find  that  its  interior  parts  resist  the  action  of  the  knife  more 
vigorously  than  does  its  outer  part.  The  fact  is  that  every 
such  pebble  is  undergoing  a  process  of  decay  from  the  attack 
of  the  atmospheric  agents.  The  proportion  of  the  decay  in 
any  given  period  depends  upon  the  size  of  the  fragment.  It 
is  relatively  greater,  the  smaller  the  mass  is. 

On  the  tolerably  smooth  rock  such  as  we  may  find  in, 
many  mountain-peaks,  atmospheric  decay  has  a  very  small 
surface  to  operate  upon.  It  works  along  on  the  exposed  face 
of  the  rock  and  on  the  open  joints  which  extend  for  a  cer- 
tain distance  below  that  surface.  If  we  assume  that  these, 
joints  cross  each  other  at  distances  of  three  feet  apart,  and 
that  there  is  a  plane  of  jointing  parallel  to  the  surface,  we 
assume  that  the  mass  is  divided  into  cubes  of  a  yard  in 
dimension,  and  that  they  each  expose  therefore  fifty-four 
square  feet  to  the  agents  of  decay.  If  we  break  the  mass  into 
fragments  each  a  cubic  foot  in  size,  we  increase  the  surface 
accessible  to  erosion  to  one  hundred  and  sixty-two  square  feet. 
If  we  break  it  still  further  into  cubes  an  inch  in  diameter,  the 
surface  exposed  to  decay  increases  to  over  a  thousand  square 
feet  of  area;  or,  in  other  words,  the  rate  at  which  the  mate- 
rial breaks  into  the  finely  divided  matter  we  term  soil,  is 
increased  more  than  one-hundred-fold  of  what  it  was  when 
the  rock  was  in  masses  three  feet  in  diameter.  Now,  most 
rocks  while  they  are  violently  precipitated  down  a  mountain- 
side undergo  a  process  of  fracture  which  in  many  cases  may 


THE  ORIGIN  AND  NATURE  OF  SOILS.  309 

serve  to  break  the  mass  into  fine  bits.  This  is  especially  the 
case  with  materials  which  have  for  a  long  time  remained 
exposed  to  the  action  of  the  weather.  Thus  in  a  moment's 
journey  in  the  avalanche  of  snow  or  stones  a  mass  of  mate- 
rial, as  great  as  the  soil  on  a  considerable  field,  maybe,  has 
advanced  a  great  ways  towards  the  degree  of  comminution 
necessary  for  its  conversion  into  fertile  earth. 

Aee  after  aee  these  mountain-rido^es,  which  rise  under  the 
impulse  of  those  deep-seated  forces  which  crumple  the  rocks, 
are  constantly  yielding  great  supplies  of  debris  to  the  torrents. 
The  ordinary  visitor  to  the  highlands  has  little  opportunity  of 
witnessing  this  process,  for  the  reason  that  it  usually  goes  on 
in  seasons  when  travellers  do  not  resort  to  the  mountain 
heights.  Such  rock-falls  commonly  occur  in  the  early  spring, 
when  the  snows  are  melting  and  the  rocks  released  from  their 
bondage  in  the  ice,  or  they  come  in  times  of  great  rain,  when 
they  are  equally  likely  to  escape  observation.  Every  one  who 
has  spent  much  time  in  Alpine  heights  has  had  occasion  to 
remark  such  falls,  and  may  often  have  narrowly  escaped  their 
dangers.  Most  commonly,  these  tumbles  are  of  small  fragments. 
A  bit  of  stone  a  few  inches  in  diameter  frequently  skips 
from  some  cliff  and  goes  bounding  down  the  slope  until  it  has 
attained  the  torrent-bed,  or  until  it  is  shivered  into  fine  frag- 
ments. Occasionally,  however,  vast  rock-falls  have  been  ob- 
served in  movement,  and  every  Alpine  valley  abounds  in  heaps 
of  stone  which  the  trained  eye  recognizes,  despite  the  vegeta- 
tion which  mantles  them,  as  the  remainders  of  such  great  acci- 
dents. 

For  the  reason  that  the  Alps  of  Switzerland  are  the  most 
inhabited  of  any  great  mountains  in  the  world,  we  have  from 
them  the  best  records  of  such  catastrophes,  which   have   often 


3IO  ASPECTS    OF  THE  EARTH. 

proved  singularly  destructive  to  life  and  property.  A  number 
of  these  accidents  have  occurred  in  modern  times.  Of  these, 
one  of  the  most  picturesque  occurred  in  the  last  century,  in 
the  high  circus-shaped  valley  which  lies  at  the  foot  of  the 
Diablerets  Mountains.  This  elevated  and  beautiful  valley  is 
of  such  difficult  access  and  at  the  same  time  affords  such  good 
pasturage  that  the  people  of  the  villages  below  have,  at  very 
great  cost,  hewn  pathways  in  the  precipices  to  gain  access  to 
it  for  their  flocks  and  herds.  Forming  the  north  side  of  the 
Diablerets  Mountain  is  the  great  precipice,  on  the  summit  of 
which  stand  several  great  mountain-peaks.      The  beds  of  rock 


Diagram  Showing  Conditions  of  a  Great  Landslide. 
A,  Mountain  of  stratified  rock.     B,  B,  Clay  layer.     C,  C,  Position  of  dSbris  after  movement. 

composing  these  masses  have  a  general  inclination  towards  the 
valley.  Although  the  most  of  the  layers  of  rock  are  of  very 
hard  material,  there  are  some  which  are  permeable  to  water. 
The  result  is  that,  from  time  to  time  in  the  past,  vast  masses 
of  cliff  have  been  detached  and  hurled  into  the  valley  below, 
where  their  course  of  ruin  can  be  traced  across  the  grassy  slope 
by  the  confused  heaps  of  stone. 

The  greatest  of  these  accidents  and  the  last  came  in  the 
night  time,  when  one  of  the  largest  of  the  peaks  which  crowned 
the  great  precipice  leaped  from  its  base  and  poured  downwards 
in  a  vast  mass  of  crumbling  stone,  which  formed  an  avalanche 
havino-   an   average   width    of  path  of  more  than  a  third  of  a 


THE   ORIGIN  AXD   NATURE  OF  SOILS,  31  I 

mile,  and  extended  to  a  distance  of  about  three  miles  from  the 
base  of  the  mountain.  Its  way  lay  through  the  most  fertile 
portion  of  the  amphitheatre,  in  which  were  many  of  those 
shelters  in  which  the  lonely  cattle-herders  of  the  valley 
protect  themselves  and  their  milch  cows.  As  the  accident 
happened  after  sunset,  a  score  or  more  of  the  herdsmen  were 
overwhelmed  along  with  their  cattle.  There  is  a  story  told  in 
connection  with  this  accident  which  has  a  peculiarly  pathetic 
side.  One  of  these  shelters,  which  housed  a  herdsman  and  his 
herd,  was  not  crushed  in  by  the  avalanche.  Its  stout  walls 
and  stronsf  roof,  built  to  resist  ofreat  falls  of  snow,  remained 
intact.  Finding  himself  thus  imprisoned,  the  unhappy  man 
labored  in  the  darkness  in  making  a  burrow  upward  towards 
the  light  of  day.  He  fed  his  cattle  with  the  stores  of  food 
which  the  place  contained,  watered  them  from  a  stream  which 
trickled  into  the  shelter,  and  slowly  heaped  the  earth  from  his 
tunnel  into  all  the  spare  room  which  the  building  afforded. 
At  length,  after  nearly  a  month  of  toil  in  the  darkness,  he 
escaped  to  the  light  of  day.  Pale  and  emaciated  from  his  long 
imprisonment  and  arduous  labor,  he  hastened  back  to  his 
native  village,  where,  according  to  the  story,  he  was  received 
as  one  who  had  returned  from  the  land  of  spirits.  He  was 
met  not  with  joy,  but  by  the  priest,  who,  with  "book,  bell, 
and  candle,"  exorcised  him  as  an  evil  spirit.  His  people 
denied  him  his  accustomed  place,  and  he  wandered  forth,  no 
one  knows  where.  It  was  only  after  the  tunnel  which  he  had 
made  was  discovered  that  his  friends  believed  that  they  had 
seen  something  more  substantial  than  his  spectre. 

On  the  northern  side  of  the  Alps,  in  the  canton  of  Schwyz, 
near  the  shores  of  Lake  Lucerne,  another  great  fall  took  place 
in   the  early  part  of  this  century.      Like  the  preceding,  it  was 


312  ASPECTS   OF  THE  EARTH. 

caused  by  a  bed  of  clay  lying  underneath  the  mountain-peak, 
which  stratum  became  softened  by  penetrating  water  and  so 
enabled  the  overlying  mass  to  launch  itself  into  the  valley 
below.  A  considerable  lake  was  formed  by  the  avalanche  of 
waste  which  remains  visible  to  this  day.  The  strong  wind 
arising  from  the  compression  of  the  air  in  front  of  the  down- 
ward rush  of  earth  and  stones,  which  is  always  a  conspicuous 
feature  in  these  great  mountain  falls,  was  especially  noted  in 
this  catastrophe.  It  was  related  to  me  by  the  son  of  one  of 
the  survivors  of  this  accident,  that  his  father,  in  company  with 
five  other  young  men,  was  walking  upon  the  road  in  the  path 
of  this  avalanche.  The  companions  were  going  two  by  two, 
with  considerable  intervals  between  them.  The  two  most  in 
advance  disappeared  beneath  the  moving  mass.  Two  others, 
who  were  not  overwhelmed  by  the  avalanche,  were  blown  to 
a  considerable  distance  and  their  clothing  partly  stripped  from 
their  bodies,  but  they  escaped  with  their  lives.  Those  furthest 
away  were  thrown  over  by  the  wind  of  the  avalanche,  but  not 
seriously  hurt. 

At  various  times  within  the  historic  period,  villages  have 
been  buried  beneath  these  falls.  Those  of  my  readers  who 
may  have  journeyed  afoot  up  the  Rhone  valley,  between  Ville- 
neuve  and  Aigle,  may  have  noticed  on  the  left  hand  of  the 
road  the  little  villao"e  of  Yvoro^ne,  famous  for  its  excellent  vine- 
yards  which  afford  the  best  white  wine  of  Switzerland.  A  part 
of  these  vineyards  lie  upon  the  slope  formed  by  a  great  moun- 
tain fall  which  occurred  in  the  last  century.  They  owe  their 
goodness,  it  may  be,  to  the  open  soil  formed  by  the  powdered 
stone  produced  in  the  catastrophe.  The  ruins  of  the  old  vil- 
lage are  said  to  lie  beneath  them,  and  from  time  to  time  the 
present  dwellers  on  the  site  fancy  they  hear  the  bell  of  the 


THE  ORIGIX  AXD  NATURE  OF  SOILS.  313 

buried  church  ringing  in  the  under  ground.  Accidents  of  this 
description,  which  occur  in  all  mountain  countries  from  time 
to  time,  are  most  common  in  regions  where  the  peaks  are 
steep.  They  also  happen  even  where  the  declivities  are  of 
moderate  slope  ;  conspicuous  cases  of  this  nature  are  common 
in  all  parts  of  the  Appalachian  range.  Whoever  has  visited 
the  White  Mountains  has  noticed  long  scars  upon  their  sides 
caused  by  the  so-called  slides  which  occur  there.  The  best 
known  of  these  happened  in  the  Willey  Notch  in  1821,  when, 
from  a  steep  mountain  slide  on  the  north  of  the  valley,  after  a 
period  of  long-continued  rain,  a  great  mass  of  rock  and  earth 
detached  itself  from  its  place  a  thousand  feet  above  the  valley, 
and  poured  as  an  avalanche  to  the  base.  An  unhappy  family, 
whose  dwelling  lay  directly  beneath  the  seat  of  the  accident, 
Med  at  the  sound  of  its  coming.  The  dwelling  they  occupied 
was  left  unharmed,  but  they  were  caught  in  their  flight  by  the 
descending  mass  and  overwhelmed. 

Although  these  incidents  are  of  a  picturesque  nature  and 
serve  but  inadequately  to  set  forth  the  character  of  the  destruc- 
tion which  these  avalanches  bring  about,  the  reader  may  gain 
from  them  some  conception  of  how  far  gravitation  gives  to  the 
mountain-torrents  a  supply  of  debris  which  may  be  ground  up 
by  their  waters  and  sent  downwards  to  the  lowlands.  But  for 
such  continuous  provisions  of  detrital  material,  these  torrents 
could  not  do  their  appointed  work.  The  most  of  the  stony 
matter  which  we  find  in  their  beds  has  come  to  them  by  the 
action  of  gravity  operating  on  steep  slopes.  Not  only  do  the 
mountains  send  down  an  occasional  supply  of  waste  in  these 
avalanches,  but  all  the  stones  which  lie  upon  the  slopes  are 
slowly  creeping  towards  the  stream -beds.  Each  winter's  frost 
expands  the  mass  and  causes  it  to  move  a  litde  way  downward. 


314 


ASPECTS   OF  THE  EARTH. 


It  may  be  that  the  onward  motion  is  only  a  small  fraction  of 
an  inch  per  year,  but  in  the  ages  this  is  sufficient  to  maintain 
a  supply  of  loose  stony  material  in  the  torrent-bed,  which  is 
ground  up  in  times  of  flood.  Where  a  torrent  is  insufficiently 
supplied  with  stony  fragments  to  maintain  a  covering  on  its 
bed,  it  eats  into  that  bed,  deepens  its  channel,  and  so  adds  to 


View  of  Stream  at  Point  where  it  Passes  from  Torrent  Section  to  River  ;    TerraL 

(Eastern  Kentucky.) 


ig  to  Form. 


the  steepness  of  the  slope  which  leads  towards  its  waters.  By 
steepening  the  slope  it  increases  the  amount  of  material  which 
is  fed  into  it.  In  this  way  almost  every  brook  has  come  to  a 
balanced  state  in  relation  to  the  supply  of  debi'is  afforded  by 
the  fields  on  either  side  of  it.  Where  the  material  comes  more 
rapidly  to  its  bed  than  its  waters  can  grind  it  up  and  remove  it, 
it  ceases  to  wear  downwards  and  remains  on  the  same  level 
until  the  sides  of  the  valley  have  somewhat  diminished  their 
declivity  by  the  process  of  erosion  ;  where,  as  before  noted,  the 


THE  ORIGIN  AND  NATURE   OF  SOILS.  315 

supply  of  detritus  is  insufficient,  the  deepening  of  the  gorge 
soon  assures  an  increased  precipitation  of  waste  into  its  bed. 

A  small  portion  of  the  detrital  matter  ground  up  in  the  beds 
of  the  torrents  is  carried  directly  downward  to  the  sea  ;  by  far 
the  greater  part  of  the  pulverized  stone  finds  its  way  in  the  low- 
lands on  to  the  sides  of  the  river,  and  forms  the  extensive  allu- 
vial plains  which  constitute  a  very  large  part  of  the  more  fertile 
soils  in  the  world.  Thus  along  the  banks  of  the  Mississippi 
and  its  innumerable  tributaries  we  find  an  area  of  detrital  plains 
composed  of  material  ground  up  in  the  torrents  of  the  hills  and 
mountains  of  the  head-waters  of  that  stream  amounting,  perhaps, 
to  more  than  fifty  thousand  square  miles.  As  soon  as  a  mountain 
brook  escapes  from  its  steep  descending  gorge  into  the  lower 
plain  region,  it  begins  to  form  such  an  alluvial  terrace.  In  each 
flood  time  the  silt-laden  waters  spread  over  the  surface  of  this 
terrace,  contributing  a  layer  of  fine  mud,  rich  in  the  possibilities 
of  nutrition  for  plants.  The  larger  the  stream,  the  wider  this 
plain.  Where  the  brook  first  emerges  upon  the  lowlands  its 
alluvial  border  lands  may  be  only  a  few  score  feet  across. 
When  it  has  become  what  we  term  a  river,  the  alluvial  deposit 
in  most  cases  is  many  times  the  width  of  the  stream  itself. 
Gradually  widening,  this  terrace  may,  as  in  the  lower  regions 
of  the  Mississippi,  attain  a  width  of  a  score  of  miles  on  either 
side  of  the  stream-bed.  At  first  this  alluvial  plain  rises  but  a 
little  way  above  the  ordinary  height  of  the  stream,  but  each 
successive  flood  adds  a  little  to  the  matter  upon  it  and  so 
increases  its  height.  At  the  same  time,  provided  the  surface  of 
the  land  does  not  change  its  level  in  relation  to  the  sea,  the  bed 
of  the  river  is  constantly  working  downward  ;  and  so  it  comes 
about  that  the  floods  visit  the  top  of  the  terrace  more  and  more 
rarely;  at  last  they  cease  even  in  the  times  of  greatest  inundation 


■2  l6 


ASPECTS    OF  THE  EARTH. 


to  attain  to  its  level.  Finally  the  terrace  may  be  left  high  up 
above  the  stream  in  the  fashion  of  those  benches  which  we 
find  along  so  many  of  our  rivers.  Those  in  the  Ashuelot  val- 
ley, a  tributary  of  the  Connecticut,  which  are  represented  in  the 
cut,  will  serve  to  illustrate  the  position  of  these  ancient  alluvial 
terraces  in  relation  to  the  present  stream. 

At  the  mouth  of  the  river  where  its  current  is  checked  by 
contact  with  the  sea,  and  where  the  salt  water  operates  to 
hasten  the  precipitation  of  these  sediments,  the  alluvial  plains 
spread  out  rapidly  on  either  side  of  its  course.  In  this  and 
the  lowermost  portion  of  the  alluvial  plains  the  course  of  the 


Diagram  Showing  Portion  of  Alluvial  Terraces  in  a  River  Valley. 
A,  A,  Bed-rock.     B,  B,  Alluvial  terraces.    C,  River  bed. 

Stream  becomes  extremely  vagarious,  and  the  variety  of  acci- 
dents serve  to  impel  it  into  new  channels.  On  the  side  of  each 
of  these  new-made  ways  it  builds  broad  plains,  and  so  by  its 
wanderings  vastly  widens  the  area  occupied  by  deposits  of  this 
nature.  At  first  these  alluvial  plains  are  very  swampy,  but  as 
the  continents  normally  tend  to  a  steadfast  growth  upward,  in 
most  cases  they  become  excellent  soils.  Even  where  the  uplift 
of  the  continent  fails  to  brino-  them  to  a  sufficient  heis^ht  to 
fit  them  for  the  uses  of  man,  it  is  often  possible  by  a  system 
of  artificial  drainage  to  win  them  to  aorriculture.  Provided  the 
inundations  which  the  river  brings  over  the  plains  do  not  occur 
during  the  tillage  season,  they  may  be  used  for  crops  without 
any  engineering  defences  whatsoever.  The  region  about  the 
mouth  of  the  Nile  is  a  capital  instance  of  those  influences  in 


THE  ORIGIN  AND  NATURE   OF  SOILS.  317 

which  the  river  floods  are  so  ordered  as  to  help  rather  than 
hinder  the  tillage  of  the  land.  The  delta  of  the  Rhine  in 
which  lies  the  greater  part  of  the  Netherlands  is  an  example 
of  what  engineering  skill  can  do  to  win  the  site  for  a  rich  state 
from  the  morasses  about  a  stream's  mouth. 

We  thus  see  that  normally  along  this  great  river  we  have 
a  region  of  soils  extending  beside  the  paths  of  the  branched 
stream  from  its  mouth  to  the  foot  of  the  torrents  in  which  a 
material  of  the  alluvial  plains  was  made.  Let  us  now  notice 
the  peculiar  advantages  connected  with  this  class  of  alluvial 
soils.  In  the  first  place,  as  may  be  readily  apprehended  from 
the  foregoing  account  of  their  formation,  these  soils  are  ex- 
tremely deep  and  are  in  most  cases  practically  of  inexhaustible 
fertility.  If  we  take  an  ordinary  specimen  of  such  soils,  and 
examine  it  closely  with  the  eye  or  better  still  with  a  powerful 
magnifying-glass,  we  see  that  it  is  composed  of  very  small 
fragments  of  stone  finely  divided  by  the  running  water  and  by 
grinding  between  the  pebbles  of  a  torrent-bed,  and  furthermore 
that  each  of  these  tiny  bits  is  more  or  less  completely  decayed. 
Looking  closely  we  may  often  see  that  the  little  grain  of  rock 
is  wrapped  around  by  the  fibres  or  rootlets  of  the  plants,  show- 
ing us  clearly  that  these  plants  are  able  to  extract  nutriment 
from  it.  Between  these  fine  grains  of  decayed  rock  we  ob- 
serve more  or  less  organic  waste, — small  particles  of  decayed 
vegetable  matter,  or  it  may  be  occasionally  a  tiny  fragment  of 
a  fresh-water  shell  which  has  been  ground  up  in  the  same  mill 
of  the  stream  and  made  to  contribute  nutriment  to  the  soil. 
Even  the  most  reckless  agriculture,  though  it  may  remove  the 
fertile  elements  from  the  superficial  layers  of  an  alluvial  ter- 
race, cannot  destroy  its  fertility  as  long  as  the  stream  can  send 
these  occasional  floods  over  the   surface.     For  each  of  these 


3i8  ASPECTS   OF  THE  EARTH. 

floods,  when  the  waters  are  checked  in  their  motion  as  they  are 
when  spread  out  as  a  thin  layer  over  the  plain,  sends  down  on 
to  the  soil  a  coating,  it  may  be  several  inches  in  thickness,  of 
fine  sediment,  alike  in  fertility  to  that  possessed  by  the  soil 
before  it  was  touched  by  man. 

We  thus  see  that  the  system  of  erosion  which  goes  on  in  the 
highlands  provides  the  lowlands  with  one  class  of  peculiarly 
fertile  soils.  There  are  certain  other  advantages  connected 
with  these  soils  of  the  alluvial  plain  which  deserve  note.  They 
are  always  singularly  open  to  the  plough,  there  are  usually 
no  stones  in  them  such  as  made  the  winning  of  the  stubborn 
fields  of  New  England  to  agriculture  a  matter  of  the  greatest 
difficulty.  Moreover,  their  products  are  in  most  cases  adjacent 
to  navigable  water-ways,  and  may  thus  enter  readily  into  the 
paths  of  commerce. 

It  is,  therefore,  not  surprising  that  nearly  all  the  first  seats 
of  agriculture  were  along  the  banks  of  rivers,  and  that  to  this 
day  the  most  fertile  fields  and  the  most  prosperous  agriculture 
lie  upon  their  borders.  These  alluvial  plains  gave  the  early 
races,  with  their  simple  primitive  means  of  agriculture,  an 
opportunity  of  passing  from  the  necessarily  savage  state  of  the 
huntsman  to  the  more  affirmed  relations  to  the  earth  which 
agriculture  brings  about.  They  are  thus  the  cradles  of  civili- 
zation,  as  they  remain  to  this  day  the  principal  seats  of  culture 
and  prosperity. 

A  second  class  of  soils  is  found  on  the  surfaces  beyond  the 
reach  of  the  silt-laden  waters  in  the  time  of  flood,  such  as  we 
may  observe  over  the  larger  part  of  any  ordinary  valley  which 
lies  beyond  the  field  possessed  by  the  continental  glaciers  of 
the  last  ice  age.  On  these  surfaces  the  declivity  is  too  small 
to  help  gravitation  to  urge  detrital  matter  down  the  slope  and 


THE   ORIGIN  AND  NATURE  OF  SOILS.  319 

into  the  mill  of  the  stream.  No  careful  study  has  as  yet  been 
made  concerning;  the  slope  on  which  this  downward  motion  of 
the  rocky  waste  towards  the  streams  takes  place.  From  some 
observations  which  I  have  made  in  the  valley  of  the  Ohio  I 
am  inclined  to  believe  that  wherever  the  slope  of  the  surface 
exceeds  ten  degrees  in  declivity  the  winter  frosts  operate  with 
a  certain  measure  of  effect  to  bring  the  detritus  worn  away 
Irom  the  rocky  slopes  into  the  control  of  the  torrents,  but  that 
where  the  declivity  is  less  considerable  the  amount  of  such 
movement  is  so  small  that  it  may  be  left  out  of  consideration. 
Over  the  most  of  a  river  basin  such  as  that  of  the  Ohio  the 
slopes  of  the  surface  do  not  exceed  five  degrees  of  inclination, 
and  on  such  areas  the  wasting  of  the  soil  is  mainly  brought 
about  by  the  direct  cutting  action  of  the  streamlets,  or  by  the 
percolation  of  the  waters  through  the  mass  of  earth  and  the 
rocks  beneath  it.  These  percolating  waters  dissolve  a  portion 
of  the  material,  and  bear  it  away  to  the  streams  in  a  state  of 
complete  solution.  Therefore  on  all  those  uplands  of  gentle 
slope  which  are  unvisited  by  flood  waters,  the  soil  is  immedi- 
ately derived  from  the  rocks  beneath  its  surface. 

The  process  by  which  these  soils  of  immediate  derivation 
are  formed  is  very  interesting.  To  see  its  nature  we  must 
make  a  section  extending  from  the  surface  downward  into  the 
solid  rocks  below.  It  is  best  we  should  make  this  section  at 
two  different  points  :  at  one  in  a  virgin  forest  which  has  not 
yet  felt  the  hand  of  man  ;  in  the  other,  in  an  old  field  adjoin- 
ing such  forest,  where  the  plough  and  the  other  destructive 
instruments  of  agriculture  have  brought  about  their  normal 
effects.  In  our  section  through  soil  beneath  a  forest  we  find 
first  a  thick  layer  often  a  foot  or  two  in  depth  composed  alto- 
gether of  decayed  vegetable  matter.     The  greater  part  of  this 


320  ASPECTS   OF  THE  EARTH. 

material  in  tlie  level  next  the  air  is  composed  of  what  we 
recognize  at  once  as  decayed  leaves,  branches  and  trunks  of 
trees.  As  we  ^o  downward  this  remainder  of  veo^etable 
structure  in  the  mass  slowly  disappears,  and  at  the  base  we 
have  a  very  fine  earthy  or  loamy  matter  the  grains  of  which 
are  almost  impalpably  small,  hardly  gritting  between  the  teeth 
when  we  test  it  in  that  manner.  This  material  is  mainly  com- 
posed of  the  ash  of  woody  fibres,  such  ash  as  we  find  in  the 
fireplace  where  wood  is  burned,  only  with  the  greater  portion 
of  the  alkaline  matter  leached  away  by  the  rain.  From  this 
upper  layer  which  is  the  product  of  vegetable  matter  falling  to 
the  surface,  we  pass  rather  suddenly  into  the  portion  of  the 
soil  where  ves^etable  matter  also  abounds  but  where  this  mate- 
rial  is  mainly  derived  from  the  decayed  roots  of  trees  and 
other  plants  which  penetrate  into  the  earth. 

In  many  cases  the  division  between  what  we  may  call  the 
aerial  layer  of  the  soil  or  that  made  by  the  falling  of  vegetable 
matter,  and  the  true  subterranean  soil,  is  indistinct.  There  are 
many  accidents  which  serve  to  confuse  the  two  layers,  the 
principal  of  which  is  the  overturning  of  trees.  Almost  all 
forests  are  liable  to  occasional  hurricanes  which  lay  the  trees 
over  thousands  of  acres  of  area  in  a  great  swath,  as  the 
scythe  fells  the  grass.  A  large  part  of  these  trees,  owing  to 
their  strong  trunks,  do  not  break  off,  but  uproot,  lifting  a 
great  sheet  of  earth  into  a  vertical  position.  As  the  dead  tree 
decays,  this  overturned  earth  tumbles  back  in  confusion  upon 
the  surface,  and  so  in  course  of  time  the  trees  over  a  wide 
area  may  be  so  frequently  uprooted  that  something  like  a 
ploughing  of  the  soil  is  accomplished,  the  overturning  action 
extending  to  an  even  greater  depth  than  is  accomplished  by 
the  work  of  our  ordinary  instruments  of  tillage.     Then   again 


THE  ORIGIN  AND   NATURE   OF  SOILS.  32 1 

the  animals  of  a  wood  bring  about  a  vast  overturning  of  the 
soil.  Darwin  has  shown  that  our  earth-worms  take  from  the 
depth  and  bring  to  the  surface  of  the  soil  a  very  large  amount 
of  material,  so  that  in  the  course  of  a  few  centuries  the  soil  is 
to  a  great  extent  overturned  by  their  action.  Our  ants,  as  my 
own  observations  have  shown,  are  in  man)-  of  our  soils,  par- 
ticularly those  of  a  sandy  nature,  even  more  effective  agents  in 
overturning  the  earth  than  the  worms.  A  large  part  of  the 
sandy  fields  in  eastern  Massachusetts  are  curiously  tilled  by 
these  animals.  In  the  course  of  a  single  season  they  may 
bring  to  the  surface  of  the  soil  as  much  as  is  equivalent  to  a 
layer  of  one-fourth  of  an  inch  in  depth  over  wide  areas.  In  a 
less  conspicuous  but  still  important  way  our  ordinary  under- 
ground mammals,  the  gophers,  groundhogs,  moles,  field  mice, 
and  sundry  other  animals,  are  engaged  in  the  same  work  of 
commingling  the  upper  and  lower  layers  of  the  soil  in  a  way 
which  greatly  promotes  the  formation  of  the  deposit  and  its 
fitness  for  the  needs  of  the  plants.  The  depth  of  this  natural!)- 
tilled  area  of  the  soil  varies  greatly,  but  in  general  it  extends 
to  more  than  a  foot  below  the  base  of  what  we  may  term 
the  aerial  stratum,  and  the  whole  of  the  section  in  which  the 
natural  overturning  agents  have  commingled  the  soil  materials 
is  often  as  much  as  three  feet  in  thickness.  The  thickness  of 
this  tillable  or  more  nutritious  stratum  of  the  earth  varies 
according  to  circumstances  which  we  have  shortly  hereafter  to 
note. 

Below  the  level  of  what  is  commonly  termed  soil  we  pass 
into  a  section  which  usually  receives  the  name  of  sub-soil.  The 
striking  difference  between  the  soil  proper  and  sub-soil  con- 
sists first  in  the  degree  of  compaction  of  the  material  in  the  two 
layers,  and  second   in   the  extent  to  which  the  earth  is  com- 


322  ASPECTS  OF  THE  EARTH. 

mino-led  with  fragments  which  have  become  separated  from  the 
subjacent  rock.  The  difference  in  the  compaction  of  these 
two  layers  is  easily  explained.  It  is  manifestly  due  to  three 
classes  of  actions.  First  to  the  overturning  of  the  soil  brought 
about  as  before  described  by  the  uprooting  of  trees  and  by 
the  burrowine  work  of  animals  ;  it  is  also  in  a  measure  due  to 
the  effect  of  the  roots  of  plants.  These  roots  first  extending 
in  the  form  of  delicate  fibres  in  the  soil  rapidly  expand  in  their 
growth,  and  press  the  grains  of  the  earth  aside.  Finally  when 
the  roots  decay  they  leave  spaces  which  often  remain  for  a 
long  time  as  hollow  tubes  but  which  gradually  become  filled 
with  substances  washed  into  them.  Then  again  water  pene- 
trating through  the  principal  part  of  the  earthy  matter  bears 
away  in  solution  a  considerable  amount  of  solid  matter,  leaving 
the  spaces  which  it  occupied,  vacant.  Last  of  all,  the  frost,  if 
it  enters  the  earth  as  it  often  does  to  considerable  depths, 
expands,  pushing  the  grains  upward  with  great  energy.  When 
the  water  becomes  again  molten,  the  weight  of  the  mass  is  not 
sufficient  to  return  it  to  the  original  state  of  compaction.  In 
the  sub-soil  most  of  these  agents  tending  to  give  the  mass  an 
open  structure  do  not  act.  It  therefore  remains  in  a  more 
solid  form. 

The  process  of  formation  of  sub-soil  which  in  time  is  to  be 
mero-ed  in  the  soil  proper  is  somewhat  various.  In  the  main, 
the  work  is  done  by  the  downward  penetration  of  water.  In  all 
cases  this  penetrating  water  leads  in  the  work  of  converting 
the  underlying  rock  into  soil.  As  we  before  noted,  pure  water 
has  a  very  slight  effect  in  dissolving  stony  matter,  but  as  the 
rain  falls  upon  the  forest  bed  it  enters  into  the  uppermost  zone 
of  the  soil  where  it  passes  through  a  great  quantity  of  decay- 
ing wood.     A  small  portion  of  the  water  gives  up  its  oxygen  to 


THE   0 RIG IX  AND  NATURE   OF  SOILS.  323 

convert  the  carbon  of  the  wood  into  carbonic-acid  gas;  another 
part  charged  with  this  carbonic  acid  penetrates  downward  to 
the  deeper  earth.  By  virtue  of  the  dissolved  gaseous  material, 
combined  oxygen  and  carbon,  the  water  gains  an  enormous 
increase  of  this  solvent  power  on  rocks  :  as  we  have  seen,  the 
increment  may  be  as  much  as  fifty-fold.  Passing  through  the 
soil  these  carbonated  waters  come  in  contact  with  the  sub- 
jacent rocks,  they  penetrate  into  every  crevice  and  open  up  by 
their  dissolving  action  innumerable  channels  which  lead  down 
indefinitely  into  the  depths.  If  the  rock  be  a  limestone,  this 
under-channelling  takes  place  with  great  rapidity  and  moment- 
ous effects,  producing  great  caverns  in  the  underground 
regions.  By  this  solvent  action  a  portion  of  the  underlying 
rock  is  directly  converted  into  soil. 

Wherever  a  crevice  in  the  rock  is  made,  the  roots  of  cer- 
tain trees,  especially  those  which  have  what  we  call  tap-roots, 
are  apt  to  penetrate.  After  a  lodgment  is  effected  in  the 
cranny  the  root  expands  with  its  growth,  rends  the  stones 
apart,  and  as  may  be  seen  in  many  cases  lifts  them  above 
their  bedding  into  the  plane  of  the  sub-soil.  The  tap-root 
trees,  such  as  our  hickories  and  many  other  forest  plants,  ap- 
pear to  rejoice  in  an  opportunity  to  invade  these  subterranean 
crevices.  It  is  certain  that  they  find  there  a  fresh  feeding 
ground,  one  that  has  not  already  been  searched  through 
and  through  by  other  plants  as  has  the  upper  portion  of  the 
solid  bed.  Thus  it  comes  about  that  the  roots  of  the  trees 
are  natural  ploughs,  or  rather  we  should  say  quarry  tools, 
operating  ceaselessly  to  rend  away  the  superficial  portions  of 
the  rocks,  and  to  bring  them  into  the  soil  plane.  The  work 
is  not  completed  when  the  mass  has  been  separated  from  the 
solid  earth,  for  the  enveloping  roots  project  their  fibrils  into 


,24 


ASPECTS    OF  THE  EARTH. 


every  crevice  of  the  fragment  and  so  divide  again  and  again 
the  mass  until  it  is  reduced  to  the  finest  grains  into  which  it  is 

divisible. 
This  process 
is  best  seen 
where  a  slaty 
rock    is  sub- 
j  e  c  t  e  d     to 
such    action. 
We  w  o  u  1  d 
often  find   in 
a  bit  of  slate 
which     i  s 
taken      from 
such    a   sub- 
soil, that  the 
roots    have  penetrated  be- 
tween   all  the  thni  lamince 
of  its  structure  and  spread 
out    like    lace-work    in  the 
confined  fissures.      Another 
specimen     if    carefully    dis- 
interred may  show  the  flakes 
of   the    slate    lying    a  little 
distance    apart,   but    so    far 
separated  that  they  fall  into 

Section  through  Forest  Mould  Soil  and  Sub-soil,  Showing        bltS    OU     DeiUg    tOUCheCl. 

Action   of  Roots  on   Bed   Rock.  ,  ^       , 

We  have  spoken  ot  the 
process  of  soil-making  as  perfectly  continuous.  The  reader 
may  well  ask  why  it  should  be  continuous — why  might  not 
the   soil    attain   a  certain    depth   and   then    afterwards    remain. 


THE   O RIG IX  AND   NATURE   OF  SOILS.  325 

permanently,  or  at  least  with  very  slight  changes,  in  its 
required  condition  ?  The  answer  to  this  is  simple  ;  it  needs 
hardly  to  be  made,  save  that  from  it  we  may  obtain  an 
important  lesson  as  to  the  earth's  history.  All  the  soil 
covering  of  the  earth  is  constantly  wasting  by  the  leaching 
out  of  these  materials.  Althoug-h  the  water  which  flows 
from  a  forest-clad  area  may  appear  to  contain  no  sediments, 
it  is  always  considerably  charged  with  mineral  matter  in 
the  state  of  complete  solution  dissolved  as  is  sugar  in  water. 
We  find  this  sediment  if  we  boil  away  the  water  of  the  clearest 
spring  to  a  point  of  complete  evaporation  ;  there  is  always  a 
little  sediment  left  on  the  bottom  of  the  vessel  which  is  com- 
posed of  material  a  part  of  which  has  wasted  from  the  soil 
throuo-h  which  the  water  flowed.  It  is  the  accumulation  of  this 
matter  which  forms  the  coating  on  the  bottom  of  an  ordinary 
tea-kettle,  though  that  vessel  may  be  supplied  by  water  from 
the  purest  spring.  In  general  this  waste  of  matter  is  most 
considerable  in  the  upper  portions  of  the  soil,  for  the  reason 
that  in  that  level  the  grains  of  rock  are  of  smaller  size  than 
in  the  lower  parts.  We  have  already  noted  the  fact  that  the 
smaller  a  bit  of  rock,  the  larger  its  surface  in  proportion  to  its 
mass,  and  so,  other  things  being  equal,  the  finer  grained  a  soil 
is.  the  more  rapidly  it  yields  material  in  the  condition  of  solu- 
tion to  the  water  which  passes  by  it. 

Thus  we  see  that  the  upper  surface  of  a  soil  is  always  sink- 
ing downwards,  and  the  lower  surface  is  also  sfoinof  nearer  the 
centre  of  the  earth.  Over  the  greater  portion  of  the  earth's 
surface  it  is  evident  that  the  adjustment  between  the  rate  of 
downward  going  in  these  two  surfaces  is  singularly  well  ac- 
complished. It  is  easy  to  see  that  where  the  upper  surface 
works   downward   b}-  the  process   of  decay  more  rapidh'  than 


o 


26  ASPECTS   OF  THE  EARTH. 


the  lower  portion  penetrates  into  the  earth,  we  should  in  time 
have  no  soil  whatever.  On  the  other  hand,  where  the  lower 
surface  of  the  soil  advances  towards  the  earth's  interior  more 
rapidly  than  the  upper  part,  that  soil  would  have  an  indefinite 
and,  in  most  cases,  a  very  great  depth.  It  is  a  familiar  fact 
that,  in  a  general  way,  the  soils  on  the  earth's  surface  are  rarely 
less  than  a  foot  in  depth  and  seldom  exceed  four  times  that 
thickness.  This  shows  us  that  there  is  some  adjustment  of 
relations  between  the  conditions  which  lead  to  the  descent  of 
these  two  forces. 

This  interesting  adjustment  is  doubtless  accomplished  in 
the  main  through  the  action  of  the  roots  themselves.  Where 
the  soil  is  thin  they  are  forced  to  take  every  opportunity  of 
penetrating  into  underlying  rock,  and  so  greatly  aid  in  rending 
it.  Where,  however,  the  lower  rock  decays  rapidly  without 
the  intervention  of  root  action,  they  satisfy  their  needs  in  the 
thicker  soil  which  forms  upon  it.  Moreover,  the  deeper  the 
soil  the  more  the  dissolving  power  of  the  water  is  expended  in 
the  upper  part  of  it.  Thus  the  balance  is  effected  which  keeps 
the  soil  generally  thin  enough  for  the  roots  of  the  plants  which 
need  the  most  nutrition  to  push  downward  into  the  earth  to 
the  measure  required  by  their  necessities,  and  at  the  same 
time  the  deposit  rarely  becomes  so  thin  as  to  make  vegetation 
impossible.  This  balance,  which  retains  soils  within  a  narrow 
limit  of  depth,  is  of  importance  ;  for,  if  the  deposit  were  in- 
definitely deep,  there  would  be  the  risk  that  the  upper  portions 
might  become  exhausted  by  the  downward  leaching  of  the 
soluble  materials  to  a  degree  that  would  be  dangerous  to  plant 
life. 

The  reader  is  now  prepared  to  understand  that  the  soil- 
coating  on  the  surface  of  our  lands   represents  the  residue  of 


THE   ORIGIN  AND   NATURE   OF  SOILS.  327 

material  which  has  remained  above  the  level  of  the  sea  durin 


t. 


a  very  long  period  of  atmospheric  erosion.  Any  one  cubic 
loot  of  soil  taken  from  the  region  of  this  continent  south  of  the 
glacial  belt — as,  for  instance,  in  the  valley  of  the  Tennessee 
River — is  likely  to  contain  in  it  bits  of  rocky  matter  from 
strata  which  have  long  disappeared  from  the  region  in  which 
we  find  them.  A  single  cubic  foot  of  material  may  represent 
the  waste  of  perhaps  a  thousand  feet  of  rocks  which  have  gone 
from  the  land  to  the  sea.  We  can  often  prove  this  proposi- 
tion by  a  close  inspection  of  the  material.  Thus,  in  the  head 
■\\'^aters  of  the  Green  River,  in  Kentucky,  I  have  found  in  the 
soil  of  the  hill-tops  small  silicified  fossils,  which  were  originally 
bedded  in  rocks  that  lay  several  hundred  feet  above  the  present 
level  of  the  surface.  These  rocks  have  entirely  gone  away  to 
the  alluvial  plains  below,  or  to  the  sea,  but  these  resistino-  bits 
have  followed  the  descending  soil  downward,  remaining  as  wit- 
nesses of  former  geological  conditions.  We  must,  therefore, 
picture  to  ourselves  the  soil-coating  of  the  earth  as  slowly 
descending  from  great  heights  towards  the  level  of  the  sea. 
But  for  the  fact  that  the  continents  are  constantly  growing 
upwards,  all  these  soil-covered  districts  would  in  the  end  be 
brought  too  near  the  level  of  the  ocean  surface  and  converted 
in  the  foundations  of  orreat  morasses. 

A  third  group  of  soils  is  found  in  regions  which  have 
recently  been  visited  by  extensive  glaciers.  We  have  already 
seen  that  where  a  region  is  exposed  to  the  normal  conditions 
of  the  atmosphere,  its  soil-coating  is  divided  into  two  distinct 
districts,  that  of  alluvial  lands  in  which  the  soil  is  derived  from 
a  considerable  distance  up  the  stream  on  which  it  borders,  and 
those  which  lie  remote  from  the  streams  where  the  soil  is 
immediately  derived  from   the  rocks   on  which   it  lies.      In  the 


328  ASPECTS   OF  THE  EARTH. 

glaciated  districts  we  have  yet  another  condition  of  affairs. 
Except  so  far  as  the  streams  recently  released  from  the  bond- 
age of  ice  have  constructed  alluvial  plains  along  their  margins, 
the  whole  of  the  glaciated  district  has  soils  which  have  neither 
been  derived  from  the  rocks  immediately  beneath  nor  borne 
to  their  position  by  ordinary  rivers,  but  which  have  a  third 
method  of  origin. 

The  broad  continental  ice  sheet  with  its  occasional  under- 
running  streams  of  water,  streams  which  follow  no  particular 
channel  but  sweep  over  the  surface  of  the  land,  wears  away  a 
vast  quantity  of  rock  matter.  A  portion  of  this  material  is 
borne  to  the  margin  of  the  glacier  b)'  its  forward  movement, 
and  is  there  accumulated  in  the  form  of  moraines,  vast  heaps 
of  confused  materials  any  cubic  foot  of  which  may  contain 
fragments  which  have  come  from  regions  very  far  apart  from 
each  other.  When  the  ice  melts  away,  the  stony  matter, 
pebbles,  sand,  and  mud  which  was  enveloped  in  its  mass,  drops 
upon  the  surface  and  forms  a  detrital  coating,  where  also  lie 
together  materials  which  come  from  widely  separated  fields. 
When  the  ice  goes  off,  the  plants  suited  to  the  conditions  seize 
upon  the  desert  of  broken  rocks  as  they  do  upon  all  unpos- 
sessed surfaces  of  the  dry  land,  save  the  deserts.  Very  quickly, 
far  more  quickly  than  indeed  the  process  takes  place  on  an  un- 
broken rock,  such  as  a  quarry,  this  detrital  matter  is  brought 
into  the  state  of  soil.  The  process  is  aided  by  the  fact  that  a 
very  large  amount  of  the  rocky  matter  is  in  a  finely  divided 
state.  These  soils  not  only  speedily  form,  but  have  a  certain 
measure  of  fertility  ;  the  trouble  with  them  is  that  they  are 
often  composed  to  a  great  extent  of  large  stones  so  that  the 
actual  amount  of  material  devoted  for  plant  use  is  relatively 
small.      Thus    in   many    stubborn    New   England  fields    which 


THE   ORIGIN  AND   NATURE  OF  SOILS.  329 

have  given  crops  of  corn  for  two  hundred  years,  scantily  but 
uniformly,  the  actual  soil  is  extremely  rich,  but  the  quantity 
of  it  when  we  take  away  the  pebbles  too  large  to  be  counted 
as  a  valuable  element  in  the  soil,  is  very  inconsiderable. 

Operating  on  the  glacial  soil  the  agents  of  decay  work  to 
great  advantage.  The  deposit  is  generally  thick,  and  there  is 
no  question  of  disrupting  the  bed-rock  which  is  often  a  slow 
process.  In  the  glacial  districts  this  work  was  done  during 
the  ice  time.  Therefore,  apart  from  their  stony  character,  the 
ordinary  glacial  soils,  or  at  least  those  formed  of  the  material 
left  on  the  surface  as  the  ice  melted  away,  are  commonly  of 
an  excellent  nature.  The  moraines  properly  so  called,  the 
heaps  left  near  the  ice  front  as  they  are  now  left  by  the  Swiss 
glaciers,  are  generally  unfertile  for  the  reason  that,  owing  to  the 
circumstances  of  their  accumulation,  they  have  been  washed 
over  by  streams  of  water  which  have  borne  away  the  valuable 
clay  which  is  the  finest  element  of  a  soil,  leaving  only  the  sand 
and  boulders. 

The  fertility  of  our  soils  depends  upon  the  chemical  char- 
acter of  the  rocks  which  by  their  decay  afford  these  materials. 
If  the  soil  is  derived  from  rocks  which  by  their  composition 
afford  a  wide  range  of  substances,  the  soil  is  sure  to  have  a 
good  measure  of  fertility,  unless  by  some  chance  it  is  too  fine 
grained  to  retain  the  water  necessary  for  the  plants  or  to  give 
access  to  the  air  which  has  to  penetrate  it  in  order  to  bring 
material  into  the  condition  for  the  use  of  vegetation. 

It  is  an  interesting  fact  to  note  that  soils  depend  for  their 
fertility  to  a  singularly  great  degree  upon  the  materials  placed 
in  the  underlying  original  rocks  by  the  action  of  life  which  has 
existed  in  former  geological  ages.  This  fact  is  particularly 
manifest   in   our  stratified   rocks,  but  in   those  of  a  crystalline 


330  ASPECTS   OF  THE  EARTH. 

nature  such  as  mica  schists,  or  even  in  rocks  which  have  ad- 
vanced further  towards  the  crystalhne  condition,  the  depend- 
ence on  ancient  animals  and  plants  is  also  manifested.  Thus 
in  the  famous  "blue  grass"  region  of  Kentucky,  an  area  often 
thousand  square  miles,  where  the  land  has  a  surprising  fertility 
and  endurance  for  tillage,  we  find  this  fertility  to  be  due  to 
the  presence  in  the  underlying  rock  of  abundant  remains  of 
animals.  Certain  small  shells  and  yet  more  minute  crustaceans 
have  power  of  taking  from  their  food  a  portion  of  limey  phos- 
phate which  they  build  into  their  skeletons;  when  they  die 
these  solid  parts  pass  into  the  strata  which  are  accumulating 
on  the  sea  floor.  In  this  manner,  in  the  above-named  region, 
a  number  of  beds  have  been  formed  not  usually  more  than  a 
few  inches  in  thickness,  which  by  their  decay  afford  the  soil  an 
invaluable  resource  of  phosphatic  matter.  This  is  made  avail 
of  in  the  growth  of  crrasses  and  "[■rains ;  each  orain  of  wheat 
returns  to  the  organic  state  matter  which  has  been  stored 
away  in  the  fossilized  bodies  of  minute  creatures  ever  since  the 
Silurian  age.  We  see  in  this  way  how  the  life  of  one  geo- 
logical period  helps  the  creatures  of  subsequent  ages. 

The  problem  of  furnishing  phosphatic  matter  to  our  soils  is 
the  gravest  of  all  which  man  has  to  meet,  unless  it  be  that  of 
preserving  the  soil  itself  from  destruction.  To  obtain  food  for 
himself  and  for  his  domesticated  animals,  man  is  compelled  to 
tax  the  phosphatic  resources  of  every  field  in  which  he  grows 
grains  of  any  description.  In  certain  limited  areas  as,  for  in- 
stance, those  above  noted  in  Kentucky,  the  sub-soil  wastes  with 
such  rapidity,  that  it  is  easy  to  maintain  the  quality  of  the  soil 
from  generation  to  generation  without  the  artificial  replacement 
of  this  precious  material,  but  in  by  far  the  greater  number  of 
cases  it  is  necessary  to  effect  this  replacement  by  some  form  of 


THE   ORIGIN  AND  NATURE  OF  SOILS.  33 1 

manuring.  Fortunately  for  the  interests  of  agriculture,  certain 
deposits  contain  a  very  large  amount  of  phosphatic  matter 
which   is  yielded  to  the  miners'  art. 

The  discovery  of  these  deposits  and  their  adaptation  to  the 
work  of  restoring  phosphorus  to  our  soil  constitutes,  perhaps, 
the  orreatest  advance  in  modern  aorriculture.  Through  this  art 
we  see  our  way  to  resist  the  exhaustion  of  our  soils  in  an  effec- 
tive manner  for  centuries  to  come.  Within  twenty-five  years 
this  industry  has  become  so  rapidly  developed,  that  at  the 
present,  artificial  phosphatic  manures  are  produced  in  this 
country  at  an  annual  cost  of  more  than  thirty  million  dollars; 
and  it  is  likely  that  before  the  end  of  the  century  it  will  become 
one  of  the  most  considerable  of  the  arts  in  all  countries  where 
agriculture  has.  attained  the  position  of  a  science.  All  these 
concentrated  phosphates  which  are  mined  for  agricultural  pur- 
poses are  essentially  like  those  which  naturally  contribute  to 
the  fertility  of  soils,  the  only  difference  being,  that  in  the  mined 
deposits  the  accumulations  are  more  extensive  than  those 
which   naturally  contribute   to  the  soil's  fertility. 

We  have  incidentally  spoken  of  the  effect  on  soil  due  to 
the  penetration  of  the  atmosphere  into  its  depths.  In  that 
state  of  nature,  the  machinery  to  accomplish  this  intermixture 
of  the  air  with  the  soil  is  brought  about  by  a  variety  of 
actions.  The  overturning  of  the  soil,  accomplished  by  the 
blowing  down  of  trees  and  by  the  action  of  their  roots  which 
form  cavities  when  they  decay,  into  which  they  penetrate,  is 
an  important  part  of  this  machinery.  The  action  of  burrow- 
ing animals,  which,  as  we  have  seen,  bring  about  a  constant 
overturning  of  the  soil,  is  another  means  whereby  this  aeration 
is  effected.  Yet  another  method  is  found  in  the  constant  down- 
ward penetration  of  water  into  the  soil.    This  water  takes  with 


332  ASPECTS   OF  THE  EARTH. 

it  a  certain  amount  of  air,  and,  as  it  sinks  into  the  crevices,  draws 
in  behind  it  a  considerable  amount  of  this  material.  In  these 
several  ways  the  oxygen  of  the  atmosphere,  which  brings  about 
the  decay  of  the  soil  and  its  preparation  for  the  uses  of  the 
plant,  is  constantly  carried  into  the  under  earth.  It  is  one  of 
the  difficulties  attendant  on  the  artificial  overturning  of  the 
soil  by  the  plough,  that  the  measure  of  this  penetration  of  the 
air  is  considerably  reduced.  In  the  single  overturning  of 
the  soil,  which  is  accomplished  in  the  seasons  of  the  plough,  a 
certain  amount  of  air  is  buried  in  the  soil,  and  the  subsequent 
processes  of  tillage  effect  the  same  result  in  a  moderate  degree. 
There  can  be  no  question  that  the  amount  of  air  introduced  by 
these  processes  in  the  uppermost  portion  of  the  soil  is  more 
considerable  than  it  is  in  the  state  of  nature  ;  but  although  the 
upper  few  mches  of  the  soil  is  well  aerated  by  our  artificial  pro- 
cesses, the  lower  part,  the  sub-soil  of  the  section  in  which  very 
important  work  is  done,  is  practically  excluded  from  contact 
with  the  air  by  our  process  of  tillage.  The  roots  no  longer 
penetrate  deeply,  and  the  greater  part  of  the  burrowing  ani- 
mals are  driven  away.  Moreover,  the  foot  of  the  plough, 
which  has  to  press  heavily  upon  the  sub-soil  in  order  to  over- 
turn the  earth  of  the  furrow,  soon  smears  and  compacts  the 
earth  at  a  certain  depth  into  a  hard  layer  which  excludes 
the  air  and  to  a  certain  extent  even  the  water  from  the  lower 

level. 

The  most  needed  correction  in  our  ordinary  methods  of 
tillao-e  consists  in  devices  to  avoid  these  difficulties.  Where 
the  tillage  is  by  the  means  of  the  spade,  these  evils  are  most 
effectively  avoided,  for  with  that  tool  there  is  no  tendency 
unduly  to  compact  the  sub-soil.  The  greatest  need  of  our 
modern  agriculture  is  for  some   instrument  which  will  overturn 


THE   ORIGIN  AND   NATURE  OF  SOILS.  333 

the  soil  in  a  cheap  manner  in  the  way  in  which  it  is  over- 
turned by  the  spade.  It  seems  as  though  there  should  be  no 
serious  mechanical  difficulty  in  producing  such  a  device,  and 
we  may  hope  that  when  people  come  to  see  the  singularly 
destructive  action  of  the  plough  it  may  be  replaced  by  some 
such  device.  At  present  there  are  but  two  methods  of  over- 
coming this  evil.  One  is  by  artificial  drains  laid  at  consider- 
able depths  below  the  surface.  These  drains  by  permitting 
the  rain-water  to  penetrate  to  considerable  depths  enable  it  to 
convey  air  into  the  sub-soil.  Something  also  is  accomplished 
by  means  of  sub-soil  ploughs  which  tear  up  the  lower  layer. 
This  is  a  clumsy  and  only  moderately  effective  device  for 
avoiding  the  evils  of  ordinary  ploughing ;  it  transfers  the  con- 
tact and  action  of  the  plough  to  a  lower  level  of  soil.  Unless 
the  process  is  well  managed,  the  sub-soil  plough  tends  to  bring 
to  the  surface  the  lower  materials  of  the  soil  layer  before  the 
process  of  preparation  for  the  uses  of  plants  has  advanced  to 
the  point  where  they  can  afford  much  nutrition. 

Although  the  plough  is  necessarily  a  disastrous  instrument 
in  its  effect  on  the  soil,  the  evils  which  arise  from  its  use  in- 
crease very  rapidly  as  the  depth  to  which  the  instrument  over- 
turns the  ground  diminishes.  In  our  American  agriculture  the 
plough  is  generally  so  managed  as  to  limit  the  penetration  ot 
the  roots  to  a  depth  of  eight  inches  or  less,  making  below  that 
level  the  hard-pan  before  described.  The  result  is  that  in  times 
of  heavy  rain,  the  upper  portion  of  the  soil  receives  more  water 
than  it  can  store  ;  it  becomes  a  mere  slush  which  tends  to 
slide  down  the  slopes  of  even  a  slight  declivity.  If  the  plough 
effected  its  work  to  the  depth  of  a  foot  or  more,  the  effect 
on  the  soil  would  be  much  less  disastrous.  If  we  are  to  pre- 
serve the  surface  of  the  earth   in  fit  condition   for  the    gener- 


334  ASPECTS   OF  THE  EARTH. 

ations  to  come,  we  must  either  by  law  or  by  public  opinion 
forbid  such  destructive  manacfement  of  our  soils. 

One  other  statement  needs  to  be  made  in  order  to  set  the 
soil  problem  clearly  before  the  mind  of  the  reader.  In  any  cubic 
foot  of  earth,  however  well  fitted  by  nature  or  by  tillage  for  the 
uses  of  plants,  the  quantity  of  material  which  at  any  one  time  is 
ready  for  assimilation  by  the  roots  is  very  small.  It  probably  in 
most  cases  does  not  exceed  one-thousandth  of  the  whole  mass. 
The  essential  peculiarity  of  this  soluble  part  of  the  soil  is  that 
it  is  ready  under  the  influence  of  plant  action  to  pass  into  the 
state  of  solution  and  be  absorbed  into  the  roots.  If  not  so 
absorbed  this  soluble  material  readily  enters  into  the  general 
mass  of  water  and  soil,  and  so  escapes  with  that  water  to  the 
stream  and  back  into  the  sea  whence  it  came  into  the  structure 
of  the  rocks.  The  orgeat  art  of  tillao-e,  the  art  throuijh  which 
in  time  we  may  expect  much  help  in  the  conservation  of  our 
soils,  is  to  regulate  the  amount  of  this  material  which  year  by 
year  offers  itself  to  the  uses  of  plants,  and  to  avoid  as  far  as 
maybe  the  deportation  of  this  precious  substance  by  the  under- 
ground waters. 

It  now  seems  likely,  though  as  yet  far  from  being  demon- 
strated, that  the  last  stages  of  the  process  by  which  the  lime- 
phosphate  and  certain  other  materials  used  by  plants  are 
brought  into  a  state  in  which  the  roots  can  appropriate  them 
are  accomplished  through  the  aid  of  ferments  which  are  at 
work  in  the  soil.  These  ferments  consist  of  oro-anic  forms  of  a 
very  lowly  organization,  allied  to  the  mould  commonly  seen  on 
decaying  organic  matter.  If  this  opinion  should  be  verified, 
it  is  not  impossible  that  we  may,  through  it,  find  our  way  to 
methods  of  stimulating  and  controlling  the  work  of  prepa- 
ration which  fits  the  soil  to  be  the  food  of  plants. 


THE   ORIGIN  AND   NATURE   OF  SOUS.  335 

We  now  turn  from  the  geological  history  of  soils  to  con- 
sider some  of  the  more  important  aspects  of  their  relation  to 
man.  We  have  noted  some  beautiful  elements  of  compensa- 
tion or  balance  which  serve  to  maintain  the  soils  on  the  earth's 
surface  under  conditions  which  fit  them  for  the  needs  of  plants. 
But  this  fittinof  is  for  the  condition  in  which  the  soil  is  occu- 
pied  by  forests  or  other  luxuriant  natural  growths  of  vegeta- 
tion. When  so  covered  by  a  mat  of  verdure  the  soil  tends  to 
maintain  itself  by  the  forces  brought  into  operation  through 
the  plant  life.  When  man  subjugates  the  soil  to  the  needs  of 
his  industries,  he  is  compelled,  in  most  cases,  to  bring  the  soil 
into  extremely  unnatural  conditions.  For  the  greater  part  of 
his  crops  he  must  strip  the  surface  of  verdure,  and  leave  it 
exposed  during  the  period  of  the  winter  season  to  the  action 
of  the  torrential  rains  which  usually  fall  at  that  time.  The 
important  result  of  this  action  is  to  expose  the  soil  to  a  process 
of  mechanical  erosion.  We  have  only  to  observe  the  surface 
in  a  little  field  in  a  time  of  heavy  storm  to  note  that  a  percep- 
tible portion  of  its  mass  is  borne  away  to  the  streams  and 
thence  to  the  lower  plains  of  the  drainage,  or,  it  may  be, 
directly  into  the  sea.  In  most  cases  in  this  country,  at  least, 
the  surface  of  our  constantly  tilled  fields  is  by  this  action  sink- 
ing down  at  the  rate  of  several  inches  in  a  century.  If  the 
under  level  of  the  soil  went  downward  at  the  same  rate  as  the 
upper,  the  result  might  not  be  immediately  disastrous ;  we 
should  have  in  effect  only  an  intensification  of  that  downward 
sinking  of  the  soil  which  we  have  seen  to  be  common  to  all 
of  them  wherever  they  lie.  But  in  all  cases  the  downward 
sinking  of  the  surface  due  to  the  waste  incident  to  tillage  takes 
place  more  rapidly  than  the  down-sinking  of  the  lower  level. 
This  is  due  to  two  reasons ;   in  the  first  place,  the  erosion  of  a 


336  ASPECTS    OF   THE  EARTH. 

tilled  field  is  practically  in  all  cases  vasdy  greater  than  the 
erosion  of  such  a  field  when  forest  or  grass-clad ;  next,  the 
removal  of  the  natural  vegetation  diminishes  the  down-sinking 
of  the  lower  level  of  the  soil  in  two  ways.  The  removal  of  the 
plants  with  strong  roots  arrests  the  process  of  disrupting  which 
they  bring  about  in  the  under  rock.  Furthermore,  the  destruc- 
tion of  the  bed  of  decayed  vegetation  so  characteristic  of  forests 
deprives  the  water  of  its  normal  charge  of  gas,  and  thus  dimin- 
ishes its  influence  in  corroding  the  bed-rocks.  It'  may  be 
accepted  as  a  general  fact  that  all  ploughed  soils  whatsoever 
are  diminishing  in  thickness  except  so  far  as  the  waste  may  be 
restored  by  contributions  of  matter  in  the  form  of  manures.  It 
is  doubtful  if  any  considerable  part  of  the  tilled  lands  in  the 
world,  at  least  those  which  are  not  visited  by  annual  floods, 
escape   a  constant  diminution   in   depth. 

This  evil  arising  from  tillage  is  in  the  alluvial  lands  prac- 
tically not  worthy  of  note,  at  least  in  those  portions  of  such 
lands  as  are  subjected  to  annual  inundations.  It  is  probable, 
however,  that  not  more  than  one-tenth  of  the  soil  value  of  the 
earth's  surface  is  contained  in  lands  of  this  nature.  The  glaci- 
ated districts,  although  they  have  in  general  soils  of  only 
moderate  fertility,  are  tolerably  exempt  from  the  evils  we  have 
noted.  The  considerable  penetrability  of  the  deposits  to  water 
makes  them  less  liable  to  wear  than  the  surface  of  our  ordinary 
fields,  and,  moreover,  the  pulverized  rocky  matter  has  in  most 
cases  a  great  depth,  and  if  any  portion  of  it  washes  away  it 
merely  lowers  the  level  of  the  soil.  The  tiller  may  be  kept 
busy  picking  out  the  large  stones,  but  with  labor,  even  where 
the  soil  washes  considerably,  he  can  maintain  its  fertility.  It 
is  difficult  to  determine  the  proportion  of  the  soil  value  of  the 
earth  which  is  contained  within  these  oflaciated  areas,  but  from 


THE   ORIGIN  AND   NATURE   OF  SOILS.  2>2>7 

such  sources  of  information  which  are  known  to  me,  I  am 
inclined  to  compute  this  area  as  not  exceeding  one-tenth  of 
the  whole.  Thus  we  have  about  two-tenths  of  the  soil  value 
of  the  earth  in  a  shape  to  be  protected  by  natural  conditions 
from  the  waste  which  man  tends  to  bring  upon  them.  The 
remaining  four-fifths  of  our  soil  are  as  far  as  they  have  been 
won  to  tillage  in  (Treat  dano-er  of  destruction. 

Unfortunately  there  are  no  statistics  at  our  con^imand 
which  serve  to  show  the  true  extent  to  which  this  wasting 
is  taking  place.  In  this,  as  in  many  other  important  mat- 
ters concerning  man's  relation  to  the  earth,  foresight  has 
not  yet  been  effectively  stimulated.  Men  look  upon  the 
earth  as  in  some  fashion  owing  them  a  living,  and  with  their 
brutal  confidence  that  it  will  continue  to  do  in  the  future  the 
part  it  has  done  by  them  in  the  past.  From  some  observa- 
tions which  I  have  made  in  the  State  of  Kentucky  I  am 
inclined  to  believe  that  the  soils  of  that  area,  though  on  the 
whole  not  more  ill-used  than  the  average  tilled  fields  of  this 
country,  are  losing  depth  at  the  rate  of  several  inches  in 
a  century.  In  Europe,  where  the  tillage  is  of  more  careful 
character,  and  where  certain  precautions  to  which  we  are  about 
to  refer  are  taken,  though  on  the  whole  imperfectly,  the  rate 
is  less.  In  Southern  Europe,  however,  since  the  beginning  of 
the  Christian  era  many  regions  which  were  once  fertile  have 
by  the  washing  away  of  their  soils  been  reduced  to  wastes 
of  bare  rock.  This  ruin  has  not  only  affected  the  tilled 
grounds  of  the  valleys,  but  has  been  even  greater  on  the 
mountainous  uplands  which,  though  of  inconsiderable  thick- 
ness, once  bore  luxuriant  forests.  The  stripping  away  of 
these  woods  has  left  the  soils  a  pre\'  to  torrential  rains  :  they 
have  been  precipitated  into  the  water-courses  and  swept  away 


338  ASPECTS   OF   THE  EARTH. 

to  the  sea  or  lodged  on  the  alluvial  lands  which  needed  no 
such  contributions  to  their  fertility. 

The  present  age  is  marked  by  a  strong  conviction  that 
man  owes  much  consideration  not  only  to  his  fellows,  but  to 
the  generations  to  come.  With  this  increase  in  the  sense  of 
duty  which  men  set  before  their  eyes  we  may  hoj)e  in  time  for 
the  most  careful  preservation  of  our  soils  whicli  is  consistent 
with  their  utilization.  We  may  soon  expect  to  see  the  law 
recognize  the  fact  that  a  man  has  only  a  right  to  use  a  portion 
of  the  earth's  surface  in  such  a  manner  as  is  necessary  for  his 
immediate  needs,  care  being  taken  that  the  reversions  of  the 
generations  to  come  have  been  properly  guarded.  When  this 
view  finds  fit  expression  in  our  laws  we  may  expect  certain  stern 
limits  to  be  put  to  the  present  reckless  waste  in  the  heritage 
of  life  represented  in  our  soils.  We  may  expect  that  all  fields 
p  from  which  the  soil  is  likely  to  be  removed  by  careless  tillage 
will  be  kept  in  the  state  of  forest  or  in  grass  lands,  in  either  of 
which  conditions  they  may  be  maintained  in  most  cases  with- 
out any  perceptible  detriment.  If  such  a  system  were  now 
enforced  all  over  the  earth  ;  if,  on  the  basis  of  well-made  sur- 
veys, the  law  prescribed  the  nature  of  the  care  to  be  taken  of 
areas  likely  to  lose  their  soils,  it  is  not  probable  that  the  result 
would  be  any  considerable  reduction  in  the  amount  of  food 
products.  A  large  part  of  the  earth's  surface  now  in  the  state 
of  forest  is  of  such  a  character  that  there  is  no  necessity 
for  that  covering  in  order  to  preserve  the  soil  from  waste. 
These  forest-covered  plains,  if  given  to  agriculture,  would 
ensure  a  maintenance  of  a  food  supply.  Even  where  the  soil 
of  steep  hillsides  needs  to  be  tilled  for  certain  peculiar  crops — 
as,  for  instance,  for  vineyards — it  is  possible  to  preserve  it  from 
destruction  by  a  system  of  terraces,  returning  the  waste  which 


THE  ORIGIN  AND  NATURE  OF  SOILS.  339 

may  find  its  way  to  the  lowlands  back  to  the  place  whence  it 
came. 

In  the  conservation  of  our  soils  we  may  also  expect  much 
from  the  development  of  irrigation  systems.  Such  a  system, 
when  properly  administered,  operates  vastly  to  extend  the 
alluvial  belt  of  the  rivers  by  taking  the  water  from  the  torrents 
charged  with  a  certain  amount  of  sediment,  and  distributing  it 
over  the  surface  of  the  lands  in  such  manner  that  the  silt  is 
retained  in  the  soil.  Moreover,  the  system  of  ditches  and 
ridges  which  have  to  be  created  on  irrigated  fields  tends 
greatly  to  prevent  the  violent  washing  of  the   soil   in   times  of 


flood 


It  is  evident  that  the  soil  problem,  though  perhaps  the  most 
serious  of  all  the  physical  difficulties  which  beset  the  future  of 
man,  is  by  no  means  beyond  his  control.  We  may  find  in  it  a 
new  and  nobler  field  for  the  exercise  of  his  intelligence  and  his 
prescience  than  he  has  as  yet  secured  in  his  careless  relations 
to  the  earth. 


INDEX. 


Action  of  subterranean  water,  i66. 
Advantages  of  beginning  the  study  of  geology 

with  river  action,  143. 
JEtna,  amount  of  matter  ejected,  88. 

peripheral  cones  of,  gi. 
Air,  effect  of  change  in  weight  of  on  earth's 
crust,  10. 

circulation  of  through  caverns,  115. 

on  planet  Venus,  200. 

on  .Saturn  and  Jupiter,  200. 

influence  of  in  making  soils,  203. 

of  Mercury  and  Venus,  203. 

ancient  composition  of,  207. 

cause  of  movements  of,  212. 

circulation  of,  212. 

how  it  enters  soil,  331. 
Air  and  ocean,  contrast  between  conditions 
of,  197. 

mingling  of,  19S. 

dependence  of  organic  life  upon,  198. 

maintenance  of  temperature,  199. 
Alluvial  plains,  functions  of,  155. 
Alluvial  terraces,  150. 
Alluvium,  effect  of  plants  on,  151. 
America,   North,   rise  of    at  close    of   glacial 
period,  2. 

earthquake  wave  action  lacking  on  At- 
lantic coast  of,  22. 

tolerably  free  from  shocks,  45. 

North  and  South,  seismic  history  of,  18. 
Ancestry  of  man,  256. 

Ancient  atmospheres,  composition  of,  207. 
Ancient  view  of  earth's  stability,  2. 
Antiquity  of  man,  256. 
Atmosphere.     See  Air. 

Base-levels  of  erosion,  165. 

Blanket  of  strata  raises  temperature,  82. 

Blue  Grotto,  134. 

Bolsena,  68. 

Boston  earthquake,  1755,  29. 

Bracciano,  68. 


Breezes,  land  and  sea.  225. 
Burial  in  caverns,  118. 
Biittes,  formation  of,  174. 

California  earthquakes,  1821,  1868,  37. 
Carbonic  acid  gas,  effects  of,  291. 
Cataracts,  classification  of,  159. 
Cave-lodes,  126. 
Cavern  fossils,  119. 

mines,  127. 
Caverns,  effect  of  on  the  imagination,  98. 

classification  of,  99. 

as  dwelling-places,  117. 

as  burial-places,  I18. 

animals  in,  i  ig. 
Change  of  level  at  Subiaco,  7. 
Change    in   weight  of  atmosphere,  effect   on 

earth's  crust,  10. 
Changes  produced  by  eruption  of  Vesuvius, 

A.D.  79,  56. 
Charleston  earthquake,  1886,  31. 
Circulation  of  air  through  caverns,  115. 
Colorado  and  Ohio  Rivers  compared,  173. 
Condition  of  ocean,  effect   of  river  deposits 

on,  156. 
Construction,  methods  of,  to  diminish  shocks, 

41. 
Continents,  growth  of,  2. 
Croll,  James,  quoted,  221. 
Cyclones,  251. 
Cypress  "  knees,"  277. 

Dampier  quoted,  224. 
Daubree's  experiment,  79. 
Davis,  \V.  M.,  quoted,  249. 
Deforesting,  evil  effects  of,  259,  261. 

effects  of  on  American  rivers,  2g3. 
Deltas,  175. 

advantages  for  primitive  peoples,  176. 

influence  on  man,  176. 
Deposits  of  minerals  and  metals  in  hot-water 
caverns,  I2g. 


342 


INDEX. 


Distribution  of  forests,  conditions  controlling, 

281. 
Domes,  106. 

Dryness  of  air  in  caverns,  116. 
Dykes,  15. 
Dyke  at  Marblehead,  Mass.,  16. 

Earth,  swayings,  6. 

pulsations,  g. 

tremors,  10. 
Earth's  diameter  dependent  on  heat,  2. 

primitive    heat,    shrinkage    due    to    loss 
of,  4. 
Earthquakes  (see  also  Volcanoes),  i. 

nature  of  shocks,  5. 

volcanic  movements,  6. 

causes  of,  13. 

regions  affected  by,  17. 

efTects  on  society,  19. 

classjification  of  shocks,  20. 

Newbury,  Mass.,  1727-1740,  28. 

Massachusetts,  1855,  29. 

Charleston,  1886,  31. 

Mississippi  valley,  181 1,  31. 

Indus  delta,  1819,  32. 

of  181 1  and  1886  compared,  33. 

New  Madrid,  1811,  33. 

Californian,  36,  37. 

Lincoln,  Eng.,  1185,  39. 

Nordlingen,  Bavaria,  1510,  39. 
Effect  of  cavern  air  on  decay,  116. 
Elevation  of  continents,  165. 
Ellet,  Charles,  quoted,  189. 
Erosion,  processes  of,  148. 

of  river  channels,  157. 
Experiment,  with  boy's  marble,  12. 

with  still  basin  of  water,  12. 

Daubree's,  79. 

with  oil  on  water,  228. 

to  show  whirl  of  air,  230. 
Explanation  of  trade-wind  movement,  218. 

Fault  caverns,  128. 
Ferments,  effect  of  on  soil,  334. 
Floods,  control  of,  iSg. 
Forests,  need  of  destroying,  25S. 

soil  of,  260. 

in  relation  to  soils,  261. 

protective  effects  of,  261. 

coal  measures  of,  263. 


Forests,  processes  of  selection,  267,  285. 

North  American,  variety  of  trees  in,  276. 

advantages  arising  from  variety  o(,  276. 

effects  of  glacial  periods  on,  285. 

economic  value  of,  295. 

American,  present  condition  of,  296. 

underground  work  of,  287, 

recovery  of  land  by,  298. 
Forest  streams,  purity  of,  268. 
Fossils  in  caverns,  119. 

Galongoon,  eruption  of,  1822,  74. 
Geogra])hic  distribution  of  caverns,  122, 
Geologic  succession  of  plants,  262. 
Glacial  deposits  and  rivers,  180. 
Glacial  soils,  327,  329. 
Green  River,  Ky.,  canon  district,  loi. 
Growth  of  continents,  2. 

Heat  determines  eaitli's  diameter,  2. 
Hell  Gate,  N.  Y.,  explosion  at,  12. 
Hot-water  caverns,  123. 
Hunt,  T.  Sterry,  quoted,  209. 

Icelandic  volcanoes,  71. 
Indices  of  a  quiet  earth,  22. 
Indus  delta  earthquake,  1819,  32. 
Irrigation,  339. 

Java  district,  volcanoes  of,  95. 

Jetties,  152. 

Joints,  part  in  soil  making,  305. 

Krakatoa,  eruption  of,  1S83,  74. 

Land  depressed  by  glacial  sheet,  2. 

Lava  caverns,  137. 

[>ava  streams  obstiucting  rivers,  181. 

Lichens  in  soil  making,  303. 

Limestone  caves,  100. 

Limestones,  ways   of  forming  in   deep  seas, 

102. 
Living  animals  in  caverns,  119. 
Lodes,  cave,  126. 
Los  Angeles  earthquake,  1812,  36. 

Magdalen  Islands,  sea  caves  in,  133. 
Malayan  volcanoes,  73. 
Marblehead  dyke,  16. 
Milne,  Jolin,  quoted,  9. 


INDEX. 


343 


ISIilne,  John,  his  directions  for  protection  of 

buildings  from  shock.  41. 
Mineral  deposits  in  caverns,  126. 
Mississippi  River,  matter  carried  out  by,  95. 

problem  of,  i83. 
Moats,  154. 

Monte  Nuovo,  formation  of,  8. 
Mountain  torrents,  146. 
Movements  of  earth's  crust,  6. 

along  New  England  coast,  9. 

Names  of  streams,  historical  significance  of, 

•      268. 
Natural  bridges,  1 14. 

Newbury,  Mass.,  earthquake,  1 727-' 40,  28. 
New  England  earthquake,  1755,  29. 
New  Jersey,  rate  of  subsidence  along  coast 
of,  g. 

Ocean  waves  produced  by  earthquake,  44. 
Ohio  and  Colorado  Rivers  compared,  173. 
Organic  life,  relation  of  soils  to,  300. 
Ox-bows,  154. 

Oxygen  and  carbon,  elimination  and  replace- 
ment of,  205,  207. 

Pacific  Ocean,  volcanoes  of,  72. 

Passage  from  torrents  to  rivers,  150. 

Phosphate  matter,  334. 

Pinnacled  rocks,  23,  24,  25. 

Plant  food,  proportion  of  in  soils,  330. 

Pliny's  letters,  50. 

Powell,  J.  W.,  quoted,  165. 

Prairie  districts,  absence  of  forests  from,  2S6. 

Pumice  carried  about  by  marine  currents,  96 

Rain-drop,  history  of  a,  145. 
Recession  of  Niagara  Falls,  162. 
Regional  distribution  of  earthquakes,  17. 
Remedies  for  deforesting,  283. 
Rivers,  wanderings  of,  167. 

effect   of   changes  of   elevation    of   land 
on,   169. 

effect  of  mountain  systems  on,  170. 

geological  consequences    of    distribution 
of,  170. 

effect  of  forests  on,  186. 
River  valley,  description  of  a,  144. 
River  valleys,  change  and  destruction  of,  176. 

danger  from  reservoirs  in,  189. 


Rock  house  caves,  136. 

Rocks  laid  down  on  sea  floor,  thickness  of,  83 

.Scott,  R.  H.,  quoted,  215. 

Shafts,  105. 

Sink-holes,  103. 

Skaptar,  70. 

Soil-formation,  stages  of,  302. 

Soils,  origin  of,  301. 

glacial,  327,  329. 

origin  of  fertility  of,  329. 

natural  and  artificial  conditions  of,  331. 
Solar  heat,  effects  in  soil  making,  302. 
Solfatara,  near  Naples,  94. 
Staffa  Cave,  134. 
Stalactites,   112. 
Story  of  the  rain-drop,  145. 
Streams,  distribution  of,  169. 
Submerged  forests.  9. 
Subsidences  in  Mississippi  valley,  34. 
Subsoil,  321. 

Subterranean  water,  action  of,  166, 
Sumbowa,  74. 
Symmes's  Hole,  141. 

Temperature  of  strata  raised  by  blanketing, 
82. 
uniformity  of  in  caverns,  106. 
of  caverns,  II 5. 

conditions  of  in  past  ages,  207. 
Temple  of  Jupiter  Serapis,  8. 
Tillage,  effects  of,  336. 
Tornadoes,  232. 

Torrent  valley  and  soil  making,  307. 
Trees,  broad   and  narrow   leaved  compared, 
265. 
adaptation  to  conditions  of  environment, 
280. 
Tributary  streams,  effect  of,  152. 

Uniformity  of  action  of  earth's  machinery, 

46. 
Upland  soils,  formation  of,  308,  31S. 

Valleys  shaped  by  ice  action,  179. 
Vesuvius,  history  of,  47,  49,  61.  65. 

present  condition  of,  56. 

observations  of  eruption  of  1882,  62. 
Volcanoes.  47. 

conditions  producing,  5. 


344 


INDEX. 


Volcanoes,  Bracciano  and  Bolsena,  68. 
about  Pacific  Ocean,  72. 
geographical  position  of,  78. 
compared  with  blast  furnace,  85. 
formed  along  a  break  or  fault,  85. 
compared  with  gas  wells,  87. 
fed  from  a  wide  field,  88. 
lunar  and  terrestrial  compared,  90. 
returning  buried  water  to  the  sea,  92. 
solid    materials  contributed  to  sea    floor 

by,95- 

of  Java  district,  95. 

elements    of  organic  life  ejected  by,  97. 

restorative  effects  on  soils,  97. 
Volcanic  caverns,  138. 
Volcanic  eruptions  : 

Vesuvius,  A.  D.  79,  48. 

Skaptar,  1783,  70. 

Sumbowa,    1822,  74. 


Volcanic  eruptions  : 

Galongoon,    1822,  74. 
Krakatoa,  1SS3,  74. 
aspect  of  sky  during,  71. 
cause  of,  78,  85. 
water  present  in,  80. 

Wanderings  of  rivers,  167. 

Water  caverns,  129. 

Waterfalls,  159. 

Waves,  ocean,  produced  by  earthquakes,  45. 

Wave  caverns,  130. 

Winds,  197. 

trade,  213. 

constant,  213. 

variable,  213. 

experiments,  217. 
Wrinkling   of   earth's   crust  through  loss    of 
heat,  3. 


DEC 


^  ^  1937 


MAR  1  6  193r 


-MAR  1  6 


1962 


URL    DECi5JS79. 

0EC4    1970 


QL  ."Ij^ 


#MfJ§i^ 


Dl 


MAP  1  ^ 
D1SCHW?G£-1 


NOV  2Sm 


Form  L-9-10m-3,'27 


Rl 


SEP  1956  X  X 
SEP  1955  X  X 


II I II  mill  nil  i|ii  I' I'll  I  I  I  II  I    \   \ 
3   1158  00669  37C 


ll^^,^i;,^l^^!^:;„''AGlO^ALLIBRARyFA^,L.^ 


^^    000  751689 


